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Interaction with Oracle


Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it.

Samuel Johnson

This chapter helps you harness the power of Oracle. You learn how PL/SQL supports the SQL commands, functions, and operators that let you manipulate Oracle data. You also learn how to manage cursors, use cursor variables, and process transactions.


SQL Support

By extending SQL, PL/SQL offers a unique combination of power and ease of use. You can manipulate Oracle data flexibly and safely because PL/SQL supports all SQL data manipulation commands (except EXPLAIN PLAN), transaction control commands, functions, pseudocolumns, and operators. Also, PL/SQL conforms to SQL92, the current ANSI/ISO SQL standard.

Note: PL/SQL does not support data definition commands such as ALTER and CREATE. For an explanation and workaround, see "Using DDL and Dynamic SQL" [*].

Data Manipulation

To manipulate Oracle data, you use the INSERT, UPDATE, DELETE, SELECT, and LOCK TABLE commands. INSERT adds new rows of data to database tables; UPDATE modifies rows; DELETE removes unwanted rows; SELECT retrieves rows that meet your search criteria; and LOCK TABLE temporarily limits access to a table.

Transaction Control

Oracle is transaction oriented; that is, Oracle uses transactions to ensure data integrity. A transaction is a series of SQL data manipulation statements that does a logical unit of work. For example, two UPDATE statements might credit one bank account and debit another.

Simultaneously, Oracle makes permanent or undoes all database changes made by a transaction. If your program fails in the middle of a transaction, Oracle detects the error and rolls back the transaction. Thus, the database is restored to its former state automatically.

You use the COMMIT, ROLLBACK, SAVEPOINT, and SET TRANSACTION commands to control transactions. COMMIT makes permanent any database changes made during the current transaction. ROLLBACK ends the current transaction and undoes any changes made since the transaction began. SAVEPOINT marks the current point in the processing of a transaction. Used with ROLLBACK, SAVEPOINT undoes part of a transaction. SET TRANSACTION establishes a read-only transaction.

SQL Functions

PL/SQL lets you use all the SQL functions including the following group functions, which summarize entire columns of Oracle data: AVG, COUNT, MAX, MIN, STDDEV, SUM, and VARIANCE.

The group functions GLB and LUB are available only with Trusted Oracle. GLB and LUB return the greatest lower bound and least upper bound of an operating system label, respectively. For more information, see Trusted Oracle7 Server Administrator's Guide.

You can use the group functions in SQL statements, but not in procedural statements. Group functions operate on entire columns unless you use the SELECT GROUP BY statement to sort returned rows into subgroups. If you omit the GROUP BY clause, the group function treats all returned rows as a single group.

You call a group function using the syntax

function_name([ALL | DISTINCT] expr)

where expr is an expression that refers to one or more database columns. If you specify the ALL option (the default), the group function considers all column values including duplicates. For example, the following statement returns the sample standard deviation (s) of all values in the comm column:

SELECT STDDEV(comm) INTO comm_sigma FROM emp;

If you specify the DISTINCT option, the group function considers only distinct values. For example, the following statement returns the number of different job titles in the emp table:

SELECT COUNT(DISTINCT job) INTO job_count FROM emp;

The COUNT function lets you specify the asterisk (*) option, which returns the number of rows in a table. For example, the following statement returns the number of employees in the emp table:

SELECT COUNT(*) INTO emp_count FROM emp;

Except for COUNT(*), all group functions ignore nulls.

SQL Pseudocolumns

PL/SQL recognizes the following SQL pseudocolumns, which return specific data items: CURRVAL, LEVEL, NEXTVAL, ROWID, and ROWNUM.

Pseudocolumns are not actual columns in a table but they behave like columns. For example, you can select values from a pseudocolumn. However, you cannot insert values into, update values in, or delete values from a pseudocolumn.

You can use pseudocolumns in SQL statements, but not in procedural statements. In the following example, you use the database sequence empno_seq and the pseudocolumn NEXTVAL (which returns the next value in a database sequence) to insert a new employee number into the emp table:

INSERT INTO emp VALUES (empno_seq.NEXTVAL, new_ename, ...);

Brief descriptions of the pseudocolumns follow. For more information, see Oracle7 Server SQL Reference.

CURRVAL and NEXTVAL

A sequence is a database object that generates sequential numbers. When you create a sequence, you can specify its initial value and an increment.

CURRVAL returns the current value in a specified sequence. Before you can reference CURRVAL in a session, you must use NEXTVAL to generate a number. A reference to NEXTVAL stores the current sequence number in CURRVAL. NEXTVAL increments the sequence and returns the next value. To obtain the current or next value in a sequence, you must use dot notation, as follows:

sequence_name.CURRVAL
sequence_name.NEXTVAL

After creating a sequence, you can use it to generate unique sequence numbers for transaction processing. However, you can use CURRVAL and NEXTVAL only in a select list, the VALUES clause, and the SET clause. In the following example, you use a sequence to insert the same employee number into two tables:

INSERT INTO emp VALUES (empno_seq.NEXTVAL, my_ename, ...);
INSERT INTO sals VALUES (empno_seq.CURRVAL, my_sal, ...);

If a transaction generates a sequence number, the sequence is incremented immediately whether you commit or roll back the transaction.

LEVEL

You use LEVEL with the SELECT CONNECT BY statement to organize rows from a database table into a tree structure. LEVEL returns the level number of a node in a tree structure. The root is level 1, children of the root are level 2, grandchildren are level 3, and so on.

You specify the direction in which the query walks the tree (down from the root or up from the branches) with the PRIOR operator. In the START WITH clause, you specify a condition that identifies the root of the tree.

ROWID

ROWID returns the rowid (binary address) of a row in a database table. Recall that PL/SQL provides a datatype also named ROWID. You can use variables of type ROWID to store rowids in a readable format. In the following example, you declare a variable named row_id for that purpose:

DECLARE
   row_id ROWID;

When you select or fetch a rowid into a ROWID variable, you can use the function ROWIDTOCHAR, which converts the binary value to an 18-byte character string. Then, you can compare the ROWID variable to the ROWID pseudocolumn in the WHERE clause of an UPDATE or DELETE statement to identify the latest row fetched from a cursor. For an example, see "Fetching Across Commits" [*].

ROWNUM

ROWNUM returns a number indicating the order in which a row was selected from a table. The first row selected has a ROWNUM of 1, the second row has a ROWNUM of 2, and so on. If a SELECT statement includes an ORDER BY clause, ROWNUMs are assigned to the retrieved rows before the sort is done.

You can use ROWNUM in an UPDATE statement to assign unique values to each row in a table. Also, you can use ROWNUM in the WHERE clause of a SELECT statement to limit the number of rows retrieved, as follows:

DECLARE
   CURSOR c1 IS SELECT empno, sal FROM emp
      WHERE sal > 2000 AND ROWNUM < 10;  -- returns 10 rows

The value of ROWNUM increases only when a row is retrieved, so the only meaningful use of ROWNUM in a WHERE clause is

... WHERE ROWNUM < constant;

For example, the following condition cannot be met because the first nine rows are never retrieved:

... WHERE ROWNUM = 10;

ROWLABEL Column

PL/SQL also recognizes the special column ROWLABEL, which Trusted Oracle creates for every database table. Like other columns, ROWLABEL can be referenced in SQL statements. However, with standard Oracle, ROWLABEL returns a null. With Trusted Oracle, ROWLABEL returns the operating system label for a row.

A common use of ROWLABEL is to filter query results. For example, the following statement counts only those rows with a security level higher than "unclassified":

SELECT COUNT(*) INTO head_count FROM emp
   WHERE ROWLABEL > 'UNCLASSIFIED';

SQL Operators

PL/SQL lets you use all the SQL comparison, set, and row operators in SQL statements. This section briefly describes some of these operators. For more information, see Oracle7 Server SQL Reference.

Comparison Operators

Typically, you use comparison operators in the WHERE clause of a data manipulation statement to form predicates, which compare one expression to another and always yields TRUE, FALSE, or NULL. You can use all the comparison operators listed below to form predicates. Moreover, you can combine predicates using the logical operators AND, OR, and NOT.

ALL

Compares a value to each value in a list or returned by a subquery and yields TRUE if all of the individual comparisons yield TRUE.

ANY, SOME

Compares a value to each value in a list or returned by a subquery and yields TRUE if any of the individual comparisons yields TRUE.

BETWEEN

Tests whether a value lies in a specified range.

EXISTS

Returns TRUE if a subquery returns at least one row.

IN

Tests for set membership.

IS NULL

Tests for nulls.

LIKE

Tests whether a character string matches a specified pattern, which can include wildcards.

Set Operators

Set operators combine the results of two queries into one result. INTERSECT returns all distinct rows selected by both queries. MINUS returns all distinct rows selected by the first query but not by the second. UNION returns all distinct rows selected by either query. UNION ALL returns all rows selected by either query, including all duplicates.

Row Operators

Row operators return or reference particular rows. ALL retains duplicate rows in the result of a query or in an aggregate expression. DISTINCT eliminates duplicate rows from the result of a query or from an aggregate expression. PRIOR refers to the parent row of the current row returned by a tree-structured query. You must use this operator in the CONNECT BY clause of such a query to define the parent-child relationship.

SQL92 Conformance

In late 1992, the American National Standards Institute (ANSI) and the International Organization for Standardization (ISO) adopted the current SQL standard known informally as SQL92, which greatly extends the previous SQL standard (SQL89).

Note: SQL92 is known officially as International Standard ISO/IEC 9075:1992, Database Language SQL, which is also available as ANSI Document ANSI X3.135-1992.

SQL92 specifies a "conforming SQL language" and, to allow implementation in stages, defines three language levels:

A conforming SQL implementation must support at least Entry SQL. PL/SQL fully supports Entry SQL.


Using DDL and Dynamic SQL

This section explains why PL/SQL does not support SQL data definition language (DDL) or dynamic SQL, then shows how to solve the problem.

Efficiency versus Flexibility

Before a PL/SQL program can be executed, it must be compiled. The PL/SQL compiler resolves references to Oracle objects by looking up their definitions in the data dictionary. Then, the compiler assigns storage addresses to program variables that will hold Oracle data so that Oracle can look up the addresses at run time. This process is called binding.

How a database language implements binding affects runtime efficiency and flexibility. Binding at compile time, called static or early binding, increases efficiency because the definitions of database objects are looked up then, not at run time. On the other hand, binding at run time, called dynamic or late binding, increases flexibility because the definitions of database objects can remain unknown until then.

Designed primarily for high-speed transaction processing, PL/SQL increases efficiency by bundling SQL statements and avoiding runtime compilation. Unlike SQL, which is compiled and executed statement- by-statement at run time (late binding), PL/SQL is processed into machine-readable p-code at compile time (early binding). At run time, the PL/SQL engine simply executes the p-code.

Some Limitations

However, this design imposes some limitations. For example, the p-code includes references to database objects such as tables and stored procedures. The PL/SQL compiler can resolve such references only if the database objects are known at compile time. In the following example, the compiler cannot process the procedure because the table is undefined until the procedure is executed at run time:

CREATE PROCEDURE create_table AS 
BEGIN 
   CREATE TABLE dept (deptno NUMBER(2), ...);  -- illegal
   ...
END;

In the next example, the compiler cannot bind the table reference in the DROP TABLE statement because the table name is unknown until the procedure is executed:

CREATE PROCEDURE drop_table (table_name IN VARCHAR2) AS 
BEGIN 
   DROP TABLE table_name;  -- illegal
   ...
END;

Overcoming the Limitations

However, the package DBMS_SQL, which is supplied with Oracle7, allows PL/SQL to execute SQL data definition and data manipulation statements dynamically at run time. For example, when called, the following stored procedure drops a specified database table:

CREATE PROCEDURE drop_table (table_name IN VARCHAR2) AS
   cid INTEGER;
BEGIN
   /* Open new cursor and return cursor ID. */
   cid := DBMS_SQL.OPEN_CURSOR;
   /* Parse and immediately execute dynamic SQL statement built by
      concatenating table name to DROP TABLE command. */
   DBMS_SQL.PARSE(cid, 'DROP TABLE ' || table_name, dbms_sql.v7);
   /* Close cursor. */
   DBMS_SQL.CLOSE_CURSOR(cid);
EXCEPTION
   /* If an exception is raised, close cursor before exiting. */
   WHEN OTHERS THEN
      DBMS_SQL.CLOSE_CURSOR(cid);
      RAISE;  -- reraise the exception
END drop_table;

For more information about package DBMS_SQL, see Oracle7 Server Application Developer's Guide.


Managing Cursors

Recall from Chapter 1 that PL/SQL uses two types of cursors: implicit and explicit. PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including queries that return only one row. However, for queries that return more than one row, you must declare an explicit cursor or use a cursor FOR loop.

Explicit Cursors

The set of rows returned by a query can consist of zero, one, or multiple rows, depending on how many rows meet your search criteria. When a query returns multiple rows, you can explicitly declare a cursor to process the rows. You can declare a cursor in the declarative part of any PL/SQL block, subprogram, or package.

You use three commands to control a cursor: OPEN, FETCH, and CLOSE. First, you initialize the cursor with the OPEN statement, which identifies the result set. Then, you use the FETCH statement to retrieve the first row. You can execute FETCH repeatedly until all rows have been retrieved. When the last row has been processed, you release the cursor with the CLOSE statement. You can process several queries in parallel by declaring and opening multiple cursors.

Declaring a Cursor

Forward references are not allowed in PL/SQL. So, you must declare a cursor before referencing it in other statements. When you declare a cursor, you name it and associate it with a specific query using the syntax

CURSOR cursor_name [(parameter[, parameter]...)]
   IS select_statement;

where parameter stands for the following syntax:

cursor_parameter_name [IN] datatype [{:= | DEFAULT} expr]

For example, you might declare cursors named c1 and c2, as follows:

DECLARE
   CURSOR c1 IS SELECT empno, ename, job, sal FROM emp 
      WHERE sal > 2000; 
   CURSOR c2 IS SELECT * FROM dept WHERE deptno = 10;

The cursor name is an undeclared identifier, not the name of a PL/SQL variable. You cannot assign values to a cursor name or use it in an expression. However, cursors and variables follow the same scoping rules. Naming cursors after database tables is allowed but not recommended.

A cursor can take parameters, which can appear in the associated query wherever constants can appear. The formal parameters of a cursor must be IN parameters. Therefore, they cannot return values to actual parameters. Also, you cannot impose the NOT NULL constraint on a cursor parameter.

As the example below shows, you can initialize cursor parameters to default values. That way, you can pass different numbers of actual parameters to a cursor, accepting or overriding the default values as you please. Also, you can add new formal parameters without having to change every reference to the cursor.

DECLARE
   CURSOR c1 (low  INTEGER DEFAULT 0,
              high INTEGER DEFAULT 99) IS SELECT ...

The scope of cursor parameters is local to the cursor, meaning that they can be referenced only within the query specified in the cursor declaration. The values of cursor parameters are used by the associated query when the cursor is opened.

Opening a Cursor

Opening the cursor executes the query and identifies the result set, which consists of all rows that meet the query search criteria. For cursors declared using the FOR UPDATE clause, the OPEN statement also locks those rows. An example of the OPEN statement follows:

DECLARE
   CURSOR c1 IS SELECT ename, job FROM emp WHERE sal < 3000;
   ...
BEGIN
   OPEN c1;
   ...
END;

Rows in the result set are not retrieved when the OPEN statement is executed. Rather, the FETCH statement retrieves the rows.

Passing Parameters You use the OPEN statement to pass parameters to a cursor. Unless you want to accept default values, each formal parameter in the cursor declaration must have a corresponding actual parameter in the OPEN statement. For example, given the cursor declaration

DECLARE
   emp_name emp.ename%TYPE;
   salary   emp.sal%TYPE;
   CURSOR c1 (name VARCHAR2, salary NUMBER) IS SELECT ...

any of the following statements opens the cursor:

OPEN c1(emp_name, 3000);
OPEN c1('ATTLEY', 1500);
OPEN c1(emp_name, salary);

In the last example, when the identifier salary is used in the cursor declaration, it refers to the formal parameter. But, when it is used in the OPEN statement, it refers to the PL/SQL variable. To avoid confusion, use unique identifiers.

Formal parameters declared with a default value need not have a corresponding actual parameter. They can simply assume their default values when the OPEN statement is executed.

You can associate the actual parameters in an OPEN statement with the formal parameters in a cursor declaration using positional or named notation. (See "Positional and Named Notation" [*].) The datatypes of each actual parameter and its corresponding formal parameter must be compatible.

Fetching with a Cursor

The FETCH statement retrieves the rows in the result set one at a time. After each fetch, the cursor advances to the next row in the result set. An example of the FETCH statement follows:

FETCH c1 INTO my_empno, my_ename, my_deptno;

For each column value returned by the query associated with the cursor, there must be a corresponding variable in the INTO list. Also, their datatypes must be compatible. Typically, you use the FETCH statement as follows:

OPEN c1;
LOOP
   FETCH c1 INTO my_record;
   EXIT WHEN c1%NOTFOUND;
   -- process data record
END LOOP;

The query can reference PL/SQL variables within its scope. However, any variables in the query are evaluated only when the cursor is opened. In the following example, each retrieved salary is multiplied by 2, even though factor is incremented after each fetch:

DECLARE
   my_sal emp.sal%TYPE;
   my_job emp.job%TYPE;
   factor INTEGER := 2;
   CURSOR c1 IS SELECT factor*sal FROM emp WHERE job = my_job;
BEGIN
   ...
   OPEN c1;  -- here factor equals 2
   LOOP
      FETCH c1 INTO my_sal;
      EXIT WHEN c1%NOTFOUND;
      ...
      factor := factor + 1;  -- does not affect FETCH
   END LOOP;
END;

To change the result set or the values of variables in the query, you must close and reopen the cursor with the input variables set to their new values.

However, you can use a different INTO list on separate fetches with the same cursor. Each fetch retrieves another row and assigns values to the target variables, as the following example shows:

DECLARE
   CURSOR c1 IS SELECT ename FROM emp;
   name1 emp.ename%TYPE;
   name2 emp.ename%TYPE;
   name3 emp.ename%TYPE;
BEGIN
   OPEN c1;
   FETCH c1 INTO name1;  -- this fetches first row
   FETCH c1 INTO name2;  -- this fetches second row
   FETCH c1 INTO name3;  -- this fetches third row
   ...
   CLOSE c1;
END;

If you fetch past the last row in the result set, the values of the target variables are indeterminate.

Note: Eventually, the FETCH statement must fail to return a row; so when that happens, no exception is raised. To detect the failure, you must use the cursor attribute %FOUND or %NOTFOUND. For more information, see "Using Cursor Attributes" [*].

Closing a Cursor

The CLOSE statement disables the cursor, and the result set becomes undefined. An example of the CLOSE statement follows:

CLOSE c1;

Once a cursor is closed, you can reopen it. Any other operation on a closed cursor raises the predefined exception INVALID_CURSOR.

Using Subqueries

In this context, a subquery is a query that appears in another query. When evaluated, the subquery provides a value or set of values to the query. Subqueries are most often used in the WHERE clause. For example, the following query returns employees not located in Chicago:

DECLARE
   CURSOR c1 IS SELECT empno, ename FROM emp
      WHERE deptno IN (SELECT deptno FROM dept 
         WHERE loc <> 'CHICAGO');
         FROM emp GROUP BY deptno) t2
      WHERE t1.deptno = t2.deptno AND "STAFF" => 5;

Using a subquery in the FROM clause, the following query returns the number and name of each department with five or more employees:

DECLARE
   CURSOR c1 IS SELECT t1.deptno, dname, "STAFF"
      FROM dept t1, (SELECT deptno, COUNT(*) "STAFF"
         FROM emp GROUP BY deptno) t2
      WHERE t1.deptno = t2.deptno AND "STAFF" => 5;

Whereas a subquery is evaluated only once per table, a correlated subquery is evaluated once per row. Consider the query below, which returns the name and salary of each employee whose salary exceeds the departmental average. For each row in the emp table, the correlated subquery computes the average salary for that row's department. The row is returned if that row's salary exceeds the average.

DECLARE
   CURSOR c1 IS SELECT deptno, ename, sal FROM emp t 
      WHERE sal > (SELECT AVG(sal) FROM emp
         WHERE t.deptno = deptno)
      ORDER BY deptno;

Implicit Cursors

Oracle implicitly opens a cursor to process each SQL statement not associated with an explicitly declared cursor. PL/SQL lets you refer to the most recent implicit cursor as the "SQL" cursor.

You cannot use the OPEN, FETCH, and CLOSE statements to control the SQL cursor. But, you can use cursor attributes to get information about the most recently executed SQL statement. See "Using Cursor Attributes" [*].


Packaging Cursors

You can separate a cursor specification from its body for placement in a package. That way, you can change the cursor body without having to change the cursor specification. You code the cursor specification in the package specification using the syntax

CURSOR cursor_name [(parameter[, parameter]...)]
    RETURN return_type;

where return_type must represent a record or a row in a database table. In the following example, you use the %ROWTYPE attribute to provide a record type that represents a row in the database table emp:

CREATE PACKAGE emp_actions AS
   /* Declare cursor specification. */
   CURSOR c1 RETURN emp%ROWTYPE;
   ...
END emp_actions;

CREATE PACKAGE BODY emp_actions AS
   /* Define cursor body. */
   CURSOR c1 RETURN emp%ROWTYPE IS
      SELECT * FROM emp WHERE sal > 3000;
   ...
END emp_actions;

The cursor specification has no SELECT statement because the RETURN clause defines the datatype of the result value. However, the cursor body must have a SELECT statement and the same RETURN clause as the cursor specification. Also, the number and datatypes of select items in the SELECT statement must match the RETURN clause.

Packaged cursors increase flexibility. For instance, you can change the cursor body in the last example, as follows, without having to change the cursor specification:

CREATE PACKAGE BODY emp_actions AS
   /* Define cursor body. */
   CURSOR c1 RETURN emp%ROWTYPE IS
      SELECT * FROM emp WHERE deptno = 20;  -- new WHERE clause
   ...
END emp_actions;


Using Cursor FOR Loops

In most situations that require an explicit cursor, you can simplify coding by using a cursor FOR loop instead of the OPEN, FETCH, and CLOSE statements. A cursor FOR loop implicitly declares its loop index as a %ROWTYPE record, opens a cursor, repeatedly fetches rows of values from the result set into fields in the record, and closes the cursor when all rows have been processed.

Consider the PL/SQL block below, which computes results from an experiment, then stores the results in a temporary table. The FOR loop index c1_rec is implicitly declared as a record. Its fields store all the column values fetched from the cursor c1. Dot notation is used to reference individual fields.

-- available online in file EXAMP7
DECLARE
   result temp.col1%TYPE;
   CURSOR c1 IS
      SELECT n1, n2, n3 FROM data_table WHERE exper_num = 1;
BEGIN
   FOR c1_rec IN c1 LOOP
         /* calculate and store the results */
      result := c1_rec.n2 / (c1_rec.n1 + c1_rec.n3);
      INSERT INTO temp VALUES (result, NULL, NULL);
   END LOOP;
   COMMIT;
END;

When the cursor FOR loop is entered, the cursor name cannot belong to a cursor that was already opened by an OPEN statement or by an enclosing cursor FOR loop. Before each iteration of the FOR loop, PL/SQL fetches into the implicitly declared record, which is equivalent to a record explicitly declared as follows:

c1_rec c1%ROWTYPE;

The record is defined only inside the loop. You cannot refer to its fields outside the loop. For example, the following reference is illegal:

FOR c1_rec IN c1 LOOP
   ...
END LOOP;
result := c1_rec.n2 + 3;  -- illegal

The sequence of statements inside the loop is executed once for each row that satisfies the query associated with the cursor. When you leave the loop, the cursor is closed automatically. This is true even if you use an EXIT or GOTO statement to leave the loop prematurely or if an exception is raised inside the loop.

Using Aliases

Fields in the implicitly declared record hold column values from the most recently fetched row. The fields have the same names as corresponding columns in the query select list. But, what happens if a select item is an expression? Consider the following example:

CURSOR c1 IS
   SELECT empno, sal+NVL(comm,0), job FROM ...

In such cases, you must include an alias for the select-item. In the next example, wages is an alias for the select item sal+NVL(comm,0):

CURSOR c1 IS
   SELECT empno, sal+NVL(comm,0) wages, job FROM ...

To reference the corresponding field, you use the alias instead of a column name, as follows:

IF emp_rec.wages < 1000 THEN ...

Passing Parameters

You can pass parameters to the cursor used in a cursor FOR loop. In the following example, you pass a department number. Then, you compute the total wages paid to employees in that department. Also, you determine how many employees have salaries higher than $2000 and how many have commissions larger than their salaries.

-- available online in file EXAMP8
DECLARE
   CURSOR emp_cursor(dnum NUMBER) IS
      SELECT sal, comm FROM emp WHERE deptno = dnum;
   total_wages NUMBER(11,2) := 0;
   high_paid   NUMBER(4) := 0;
   higher_comm NUMBER(4) := 0;
BEGIN
   /* The number of iterations will equal the number of rows *
    * returned by emp_cursor.                                */
   FOR emp_record IN emp_cursor(20) LOOP
      emp_record.comm := NVL(emp_record.comm, 0);
      total_wages := total_wages + emp_record.sal +
         emp_record.comm;
      IF emp_record.sal > 2000.00 THEN
         high_paid := high_paid + 1;
      END IF;
      IF emp_record.comm > emp_record.sal THEN
         higher_comm := higher_comm + 1;
      END IF;
   END LOOP;
   INSERT INTO temp VALUES (high_paid, higher_comm,
      'Total Wages: ' || TO_CHAR(total_wages));
   COMMIT;
END;


Using Cursor Variables

Like a cursor, a cursor variable points to the current row in the result set of a multi-row query. But, cursors differ from cursor variables the way constants differ from variables. Whereas a cursor is static, a cursor variable is dynamic because it is not tied to a specific query. You can open a cursor variable for any type-compatible query. This gives you more flexibility.

Also, you can assign new values to a cursor variable and pass it as a parameter to subprograms, including subprograms stored in an Oracle database. This gives you an easy way to centralize data retrieval.

Cursor variables are available to every PL/SQL client. For example, you can declare a cursor variable in a PL/SQL host environment such as an OCI or Pro*C program, then pass it as a bind variable to PL/SQL. Moreover, application development tools such as Oracle Forms and Oracle Reports, which have a PL/SQL engine, can use cursor variables entirely on the client side.

The Oracle Server also has a PL/SQL engine. So, you can pass cursor variables back and forth between an application and server via remote procedure calls (RPCs).

What Are Cursor Variables?

Cursor variables are like C or Pascal pointers, which hold the memory location (address) of some object instead of the object itself. So, declaring a cursor variable creates a pointer, not an object. In PL/SQL, a pointer has datatype REF X, where REF is short for REFERENCE and X stands for a class of objects. Therefore, a cursor variable has datatype REF CURSOR. Currently, cursor variables are the only REF variables that you can declare.

To execute a multi-row query, Oracle opens an unnamed work area that stores processing information. To access the information, you can use an explicit cursor, which names the work area. Or, you can use a cursor variable, which points to the work area. Whereas a cursor always refers to the same query work area, a cursor variable can refer to different work areas. So, cursors and cursor variables are not interoperable; that is, you cannot use one where the other is expected.

Why Use Cursor Variables?

Mainly, you use cursor variables to pass query result sets between PL/SQL stored subprograms and various clients. Neither PL/SQL nor any of its clients owns a result set; they simply share a pointer to the query work area in which the result set is stored. For example, an OCI client, Oracle Forms application, and Oracle Server can all refer to the same work area.

A query work area remains accessible as long as any cursor variable points to it. Therefore, you can pass the value of a cursor variable freely from one scope to another. For example, if you pass a host cursor variable to a PL/SQL block embedded in a Pro*C program, the work area to which the cursor variable points remains accessible after the block completes.

If you have a PL/SQL engine on the client side, calls from client to server impose no restrictions. For example, you can declare a cursor variable on the client side, open and fetch from it on the server side, then continue to fetch from it back on the client side.

Also, you can reduce network traffic by having a PL/SQL block open (or close) several host cursor variables in a single round trip.

Defining REF CURSOR Types

To create cursor variables, you take two steps. First, you define a REF CURSOR type, then declare cursor variables of that type. You can define REF CURSOR types in any PL/SQL block, subprogram, or package using the syntax

TYPE ref_type_name IS REF CURSOR RETURN return_type;

where ref_type_name is a type specifier used in subsequent declarations of cursor variables and return_type must represent a record or a row in a database table. In the following example, you specify a return type that represents a row in the database table dept:

DECLARE
   TYPE DeptCurTyp IS REF CURSOR RETURN dept%ROWTYPE; 

REF CURSOR types can be strong (restrictive) or weak (nonrestrictive). As the next example shows, a strong REF CURSOR type definition specifies a return type, but a weak definition does not:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;  -- strong
   TYPE GenericCurTyp IS REF CURSOR;  -- weak 

Strong REF CURSOR types are less error prone because the PL/SQL compiler lets you associate a strongly typed cursor variable only with type-compatible queries. However, weak REF CURSOR types are more flexible because the compiler lets you associate a weakly typed cursor variable with any query.

Declaring Cursor Variables

Once you define a REF CURSOR type, you can declare cursor variables of that type in any PL/SQL block or subprogram. In the following example, you declare the cursor variable dept_cv:

DECLARE
   TYPE DeptCurTyp IS REF CURSOR RETURN dept%ROWTYPE;
   dept_cv DeptCurTyp;  -- declare cursor variable

Note: You cannot declare cursor variables in a package. Unlike package variables, cursor variables do not have persistent state. Remember, declaring a cursor variable creates a pointer, not an object. So, cursor variables cannot be saved in the database.

Cursor variables follow the usual scoping and instantiation rules. Local PL/SQL cursor variables are instantiated when you enter a block or subprogram and cease to exist when you exit.

In the RETURN clause of a REF CURSOR type definition, you can use %ROWTYPE to specify a record type that represents a row returned by a (previously declared) cursor variable, as follows:

DECLARE
   TYPE TmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   tmp_cv TmpCurTyp;  -- declare cursor variable
   TYPE EmpCurTyp IS REF CURSOR RETURN tmp_cv%ROWTYPE;
   emp_cv EmpCurTyp;  -- declare cursor variable

Likewise, you can use %TYPE to provide the datatype of a record variable, as the following example shows:

DECLARE
   dept_rec dept%ROWTYPE;  -- declare record variable
   TYPE DeptCurTyp IS REF CURSOR RETURN dept_rec%TYPE;
   dept_cv DeptCurTyp;  -- declare cursor variable

In the final example, you specify a user-defined RECORD type in the RETURN clause:

DECLARE
   TYPE EmpRecTyp IS RECORD (
      empno NUMBER(4),
      ename VARCHAR2(1O),
      sal   NUMBER(7,2));
   TYPE EmpCurTyp IS REF CURSOR RETURN EmpRecTyp;
   emp_cv EmpCurTyp;  -- declare cursor variable

As Parameters

You can declare cursor variables as the formal parameters of functions and procedures. In the following example, you define the REF CURSOR type EmpCurTyp, then declare a cursor variable of that type as the formal parameter of a procedure:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp) IS ...

Controlling Cursor Variables

You use three statements to control a cursor variable: OPEN-FOR, FETCH, and CLOSE. First, you OPEN a cursor variable FOR a multi-row query. Then, you FETCH rows from the result set one at a time. When all the rows are processed, you CLOSE the cursor variable.

Opening a Cursor Variable

The OPEN-FOR statement associates a cursor variable with a multi-row query, executes the query, and identifies the result set. The statement syntax is

OPEN {cursor_variable_name | :host_cursor_variable_name} 
   FOR select_statement;

where host_cursor_variable_name identifies a cursor variable declared in a PL/SQL host environment such as an OCI or Pro*C program.

Unlike cursors, cursor variables do not take parameters. No flexibility is lost, however, because you can pass whole queries (not just parameters) to a cursor variable. The query can reference bind variables and PL/SQL variables, parameters, and functions but cannot be FOR UPDATE.

In the example below, you open the cursor variable emp_cv. Notice that you can apply cursor attributes (%FOUND, %NOTFOUND, %ISOPEN, and %ROWCOUNT) to a cursor variable.

IF NOT emp_cv%ISOPEN THEN 
   /* Open cursor variable. */
   OPEN emp_cv FOR SELECT * FROM emp;
END IF;

Other OPEN-FOR statements can open the same cursor variable for different queries. You need not close a cursor variable before reopening it. (Recall that consecutive OPENs of a static cursor raise the predefined exception CURSOR_ALREADY_OPEN.) When you reopen a cursor variable for a different query, the previous query is lost.

In a Stored Procedure Typically, you open a cursor variable by passing it to a stored procedure that declares a cursor variable as one of its formal parameters. For example, the following packaged procedure opens the cursor variable emp_cv:

CREATE PACKAGE emp_data AS
   ...
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp);
END emp_data;
 
CREATE PACKAGE BODY emp_data AS
   ...
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp) IS
   BEGIN
      OPEN emp_cv FOR SELECT * FROM emp;
   END open_emp_cv;
END emp_data;

When you declare a cursor variable as the formal parameter of a subprogram that opens the cursor variable, you must specify the IN OUT mode. That way, the subprogram can pass an open cursor back to the caller.

Alternatively, you can use a standalone procedure to open the cursor variable. Simply define the REF CURSOR type in a separate package, then reference that type in the standalone procedure. For instance, if you create the following (bodiless) package, you can create standalone procedures that reference the types it defines:

CREATE PACKAGE cv_types AS
   TYPE GenericCurTyp IS REF CURSOR;
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   TYPE DeptCurTyp IS REF CURSOR RETURN dept%ROWTYPE;
   ...
END cv_types;

In the following example, you create a standalone procedure that references the REF CURSOR type EmpCurTyp, which is defined in the package cv_types:

CREATE PROCEDURE open_emp_cv (emp_cv IN OUT cv_types.EmpCurTyp) AS BEGIN 
   OPEN emp_cv FOR SELECT * FROM emp;
END open_emp_cv;

To centralize data retrieval, you can group type-compatible queries in a stored procedure. In the example below, the packaged procedure declares a selector as one of its formal parameters. (In this context, a selector is a variable used to select one of several alternatives in a conditional control statement.) When called, the procedure opens the cursor variable emp_cv for the chosen query.

CREATE PACKAGE emp_data AS
   TYPE GenericCurTyp IS REF CURSOR;
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp,
                          choice IN NUMBER);
END emp_data;
 
CREATE PACKAGE BODY emp_data AS
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp,
                          choice IN NUMBER) IS
   BEGIN
      IF choice = 1 THEN
         OPEN emp_cv FOR SELECT * FROM emp WHERE comm IS NOT NULL;
      ELSIF choice = 2 THEN
         OPEN emp_cv FOR SELECT * FROM emp WHERE sal > 2500;
      ELSIF choice = 3 THEN
         OPEN emp_cv FOR SELECT * FROM emp WHERE deptno = 20;
      END IF;
   END open_emp_cv;
END emp_data;

For more flexibility, you can pass a cursor variable and selector to a stored procedure that executes queries with different return types. Consider the following example:

CREATE PACKAGE BODY emp_data AS
   PROCEDURE open_cv (generic_cv IN OUT GenericCurTyp,
                      choice     IN NUMBER) IS
   BEGIN
      IF choice = 1 THEN
         OPEN generic_cv FOR SELECT * FROM emp;
      ELSIF choice = 2 THEN
         OPEN generic_cv FOR SELECT * FROM dept;
      ELSIF choice = 3 THEN
         OPEN generic_cv FOR SELECT * FROM salgrade;
      END IF;
   END open_cv;
END emp_data;

Using a Bind Variable You can declare a cursor variable in a PL/SQL host environment such as an OCI or Pro*C program. To use the cursor variable, you must pass it as a bind variable to PL/SQL. In the following Pro*C example, you pass a host cursor variable and selector to a PL/SQL block, which opens the cursor variable for the chosen query:

EXEC SQL BEGIN DECLARE SECTION;
   ...
   /* Declare host cursor variable. */
   SQL_CURSOR generic_cv;
   int        choice;
EXEC SQL END DECLARE SECTION;
...
/* Initialize host cursor variable. */
EXEC SQL ALLOCATE :generic_cv;
...
/* Pass host cursor variable and selector to PL/SQL block. */
EXEC SQL EXECUTE
BEGIN
   IF :choice = 1 THEN
      OPEN :generic_cv FOR SELECT * FROM emp;
   ELSIF :choice = 2 THEN
      OPEN :generic_cv FOR SELECT * FROM dept;
   ELSIF :choice = 3 THEN
      OPEN :generic_cv FOR SELECT * FROM salgrade;
   END IF;
END;
END-EXEC;

Host cursor variables are compatible with any query return type. They behave just like weakly typed PL/SQL cursor variables.

Fetching from a Cursor Variable

The FETCH statement retrieves rows one at a time from the result set of a multi-row query. The statement syntax follows:

FETCH {cursor_variable_name | :host_cursor_variable_name}
   INTO {variable_name[, variable_name]... | record_name};

In the next example, you fetch rows from the cursor variable emp_cv into the user-defined record emp_rec:

LOOP
   /* Fetch from cursor variable. */
   FETCH emp_cv INTO emp_rec;
   EXIT WHEN emp_cv%NOTFOUND; -- exit when last row is fetched
   -- process data record
END LOOP;

Any variables in the associated query are evaluated only when the cursor variable is opened. To change the result set or the values of variables in the query, you must reopen the cursor variable with the variables set to their new values. However, you can use a different INTO clause on separate fetches with the same cursor variable. Each fetch retrieves another row from the same result set.

PL/SQL makes sure the return type of the cursor variable is compatible with the INTO clause of the FETCH statement. For each column value returned by the query associated with the cursor variable, there must be a corresponding, type-compatible field or variable in the INTO clause. Also, the number of fields or variables must equal the number of column values. Otherwise, you get an error.

The error occurs at compile time if the cursor variable is strongly typed or at run time if it is weakly typed. At run time, PL/SQL raises the predefined exception ROWTYPE_ MISMATCH before the first fetch. So, if you trap the error and execute the FETCH statement using a different INTO clause, no rows are lost.

When you declare a cursor variable as the formal parameter of a subprogram that fetches from the cursor variable, you must specify the IN (or IN OUT) mode. However, if the subprogram also opens the cursor variable, you must specify the IN OUT mode.

If you try to fetch from a closed or never-opened cursor variable, PL/SQL raises the predefined exception INVALID_CURSOR.

Closing a Cursor Variable

The CLOSE statement disables a cursor variable. After that, the associated result set is undefined. The statement syntax follows:

CLOSE {cursor_variable_name | :host_cursor_variable_name);

In the following example, when the last row is processed, you close the cursor variable emp_cv:

LOOP
   FETCH emp_cv INTO emp_rec;
   EXIT WHEN emp_cv%NOTFOUND;
   -- process data record
END LOOP;
/* Close cursor variable. */
CLOSE emp_cv;

When declaring a cursor variable as the formal parameter of a subprogram that closes the cursor variable, you must specify the IN (or IN OUT) mode.

If you try to close an already-closed or never-opened cursor variable, PL/SQL raises the predefined exception INVALID_CURSOR.

Some Examples

Consider the stored procedure below, which searches the database of a main library for books, periodicals, and tapes. A master table stores the title and category code (1 = book, 2 = periodical, 3 = tape) of each item. Three detail tables store category-specific information. When called, the procedure searches the master table by title, uses the associated category code to pick an OPEN-FOR statement, then opens a cursor variable for a query of the proper detail table.

CREATE PACKAGE cv_types AS
   TYPE LibCurTyp IS REF CURSOR;
   ...
END cv_types;

CREATE PROCEDURE find_item (title  VARCHAR2(100),
                            lib_cv IN OUT cv_types.LibCurTyp) AS
   code BINARY_INTEGER;
BEGIN
   SELECT item_code FROM titles INTO code
      WHERE item_title = title;
   IF code = 1 THEN 
      OPEN lib_cv FOR SELECT * FROM books 
         WHERE book_title = title;
   ELSIF code = 2 THEN 
      OPEN lib_cv FOR SELECT * FROM periodicals 
         WHERE periodical_title = title;
   ELSIF code = 3 THEN 
      OPEN lib_cv FOR SELECT * FROM tapes 
         WHERE tape_title = title;
   END IF;
END find_item;

A client-side application in a branch library might use the following PL/SQL block to display the retrieved information:

DECLARE
   lib_cv         cv_types.LibCurTyp;
   book_rec       books%ROWTYPE;
   periodical_rec periodicals%ROWTYPE;
   tape_rec       tapes%ROWTYPE;
BEGIN
   get_title(:title);  -- title is a host variable
   find_item(:title, lib_cv);
   FETCH lib_cv INTO book_rec;
   display_book(book_rec);
EXCEPTION
   WHEN ROWTYPE_MISMATCH THEN
      BEGIN
         FETCH lib_cv INTO periodical_rec;
         display_periodical(periodical_rec);
      EXCEPTION
         WHEN ROWTYPE_MISMATCH THEN
            FETCH lib_cv INTO tape_rec;
            display_tape(tape_rec);
      END;
END;

The following Pro*C program prompts the user to select a database table, opens a cursor variable for a query of that table, then fetches rows returned by the query:

#include <stdio.h>
#include <sqlca.h>
void sql_error();
main()
{
   char temp[32];
   EXEC SQL BEGIN DECLARE SECTION;
      char * uid = "scott/tiger";
      SQL_CURSOR generic_cv;   /* cursor variable */
      int    table_num;        /* selector        */
      struct                   /* EMP record      */
      {
         int   emp_num;
         char  emp_name[11];
         char  job_title[10];
         int   manager;
         char  hire_date[10];
         float salary;
         float commission;
         int   dept_num;
      } emp_rec;
      struct                   /* DEPT record     */
      {         
         int   dept_num;
         char  dept_name[15];
         char  location[14];
      } dept_rec;
      struct                   /* BONUS record    */
      {
         char  emp_name[11];
         char  job_title[10];
         float salary;
      } bonus_rec;
   EXEC SQL END DECLARE SECTION;
   /* Handle Oracle errors. */
   EXEC SQL WHENEVER SQLERROR DO sql_error();

   /* Connect to Oracle. */
   EXEC SQL CONNECT :uid;
 
  /* Initialize cursor variable. */
   EXEC SQL ALLOCATE :generic_cv;

   /* Exit loop when done fetching. */
   EXEC SQL WHENEVER NOT FOUND DO break;

   for (;;)
   {
      printf("\n1 = EMP, 2 = DEPT, 3 = BONUS");
      printf("\nEnter table number (0 to quit): ");
      gets(temp);
      table_num = atoi(temp);
      if (table_num <= 0) break;

      /* Open cursor variable. */
      EXEC SQL EXECUTE
         BEGIN
            IF :table_num = 1 THEN
               OPEN :generic_cv FOR SELECT * FROM emp; 
            ELSIF :table_num = 2 THEN
               OPEN :generic_cv FOR SELECT * FROM dept;
            ELSIF :table_num = 3 THEN
               OPEN :generic_cv FOR SELECT * FROM bonus; 
            END IF;
         END;
      END-EXEC;

      for (;;)
      {
         switch (table_num)
         {
            case 1:  /* Fetch row into EMP record. */
               EXEC SQL FETCH :generic_cv INTO :emp_rec;
               break;
            case 2:  /* Fetch row into DEPT record. */
               EXEC SQL FETCH :generic_cv INTO :dept_rec;
               break;
            case 3:  /* Fetch row into BONUS record. */
               EXEC SQL FETCH :generic_cv INTO :bonus_rec;
               break;
         }
         /* Process data record here. */
      }
      /* Close cursor variable. */
      EXEC SQL CLOSE :generic_cv;
   }
   exit(0);
}
void sql_error()
{  
   /* Handle SQL error here. */
}

Reducing Network Traffic

When passing host cursor variables to PL/SQL, you can reduce network traffic by grouping OPEN-FOR statements. For example, the following PL/SQL block opens five cursor variables in a single round-trip:

/* anonymous PL/SQL block in host environment */ 
BEGIN 
   OPEN :emp_cv FOR SELECT * FROM emp; 
   OPEN :dept_cv FOR SELECT * FROM dept; 
   OPEN :grade_cv FOR SELECT * FROM salgrade; 
   OPEN :pay_cv FOR SELECT * FROM payroll; 
   OPEN :ins_cv FOR SELECT * FROM insurance; 
END;

This might be useful in Oracle Forms, for instance, when you want to populate a multi-block form.

When you pass host cursor variables to a PL/SQL block for opening, the query work areas to which they point remain accessible after the block completes. That allows your OCI or Pro*C program to use these work areas for ordinary cursor operations. In the following example, you open five such work areas in a single round trip:

BEGIN
   OPEN :c1 FOR SELECT 1 FROM dual;
   OPEN :c2 FOR SELECT 1 FROM dual;
   OPEN :c3 FOR SELECT 1 FROM dual;
   OPEN :c4 FOR SELECT 1 FROM dual;
   OPEN :c5 FOR SELECT 1 FROM dual;
END;

The cursors assigned to c1, c2, c3, c4, and c5 behave normally, and you can use them for any purpose. When finished, simply release the cursors, as follows:

BEGIN 
   CLOSE :c1;
   CLOSE :c2;
   CLOSE :c3;
   CLOSE :c4;
   CLOSE :c5;
END;

Avoiding Exceptions

If both cursor variables involved in an assignment are strongly typed, they must have the same datatype. In the following example, even though the cursor variables have the same return type, the assignment raises an exception because they have different datatypes:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   TYPE TmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   ...
   PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp,
                          tmp_cv IN OUT TmpCurTyp) IS 
   BEGIN 
      ...
      emp_cv := tmp_cv;  -- causes 'wrong type' error
   END;

However, if one or both cursor variables are weakly typed, they need not have the same datatype.

If you try to fetch from, close, or apply cursor attributes to a cursor variable that does not point to a query work area, PL/SQL raises the predefined exception INVALID_CURSOR. You can make a cursor variable (or parameter) point to a query work area in two ways:

The following example shows how these ways interact:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   emp_cv1 EmpCurTyp;
   emp_cv2 EmpCurTyp;
   emp_rec emp%ROWTYPE;
BEGIN
   /* The following assignment is useless because emp_cv1
      does not point to a query work area yet. */
   emp_cv2 := emp_cv1;  -- useless 
   /* Make emp_cv1 point to a query work area. */
   OPEN emp_cv1 FOR SELECT * FROM emp;
   /* Use emp_cv1 to fetch first row from emp table. */
   FETCH emp_cv1 INTO emp_rec; 
   /* The following fetch raises an exception because emp_cv2
      does not point to a query work area yet. */
   FETCH emp_cv2 INTO emp_rec;  -- raises INVALID_CURSOR
EXCEPTION
   WHEN INVALID_CURSOR THEN
      /* Make emp_cv1 and emp_cv2 point to same work area. */
      emp_cv2 := emp_cv1;
      /* Use emp_cv2 to fetch second row from emp table. */
      FETCH emp_cv2 INTO emp_rec; 
      /* Reuse work area for another query. */
      OPEN emp_cv2 FOR SELECT * FROM old_emp;
      /* Use emp_cv1 to fetch first row from old_emp table.
         The following fetch succeeds because emp_cv1 and
         emp_cv2 point to the same query work area. */
      FETCH emp_cv1 INTO emp_rec;  -- succeeds
END;

Be careful when passing cursor variables as parameters. At run time, PL/SQL raises ROWTYPE_MISMATCH if the return types of the actual and formal parameters are incompatible.

In the Pro*C example below, you define a packaged REF CURSOR type, specifying the return type emp%ROWTYPE. Next, you create a standalone procedure that references the new type. Then, inside a PL/SQL block, you open a host cursor variable for a query of the dept table. Later, when you pass the open host cursor variable to the stored procedure, PL/SQL raises ROWTYPE_MISMATCH because the return types of the actual and formal parameters are incompatible.

/* bodiless package */
CREATE PACKAGE cv_types AS
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   ...
END cv_types;

/* standalone procedure */
CREATE PROCEDURE open_emp_cv (emp_cv IN OUT cv_types.EmpCurTyp) AS
BEGIN
   OPEN emp_cv FOR SELECT * FROM emp;
END open_emp_cv;

/* anonymous PL/SQL block in Pro*C program */
EXEC SQL EXECUTE
   BEGIN
      OPEN :cv FOR SELECT * FROM dept;
      ...
      open_emp_cv(:cv); -- raises ROWTYPE_MISMATCH because emp and
                        -- dept tables have different rowtypes
   END;
END-EXEC;

Guarding Against Aliasing

Like all pointers, cursor variables introduce the possibility of aliasing. Consider the example below. After the assignment, emp_cv2 is an alias of emp_cv1 because both point to the same query work area. So, both can alter its state. That is why the first fetch from emp_cv2 fetches the third row (not the first) and why the second fetch from emp_cv2 fails after you close emp_cv1.

PROCEDURE get_emp_data (emp_cv1 IN OUT EmpCurTyp,
                        emp_cv2 IN OUT EmpCurTyp) IS
   emp_rec emp%ROWTYPE;
BEGIN 
   OPEN emp_cv1 FOR SELECT * FROM emp; 
   emp_cv2 := emp_cv1;
   FETCH emp_cv1 INTO emp_rec;  -- fetches first row
   FETCH emp_cv1 INTO emp_rec;  -- fetches second row
   FETCH emp_cv2 INTO emp_rec;  -- fetches third row
   CLOSE emp_cv1;
   FETCH emp_cv2 INTO emp_rec;  -- raises INVALID_CURSOR
   ...
END get_emp_data;

Aliasing also occurs when the same actual parameter appears twice in a subprogram call. Unless both formal parameters are IN parameters, the result is indeterminate, as the following example shows:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE; 
   emp_cv  EmpCurTyp;
   emp_rec emp%ROWTYPE;
   PROCEDURE open_emp_cv (cv1 IN OUT EmpCurTyp,
                          cv2 IN OUT EmpCurTyp) IS
   BEGIN
      OPEN cv1 FOR SELECT * FROM emp WHERE ename = 'KING';
      OPEN cv2 FOR SELECT * FROM emp WHERE ename = 'BLACK';
   END open_emp_cv;
BEGIN
   open_emp_cv(emp_cv, emp_cv);
   FETCH emp_cv INTO emp_rec;  -- indeterminate; might return 
                               -- row for 'KING' or 'BLACK'
   ...
END;

Restrictions

Currently, cursor variables are subject to the following restrictions, some of which future releases of PL/SQL will remove:

      PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp,
                             tmp_cv IN OUT TmpCurTyp) IS 
      BEGIN
         ...
         IF emp_cv = tmp_cv THEN ...  -- illegal
         IF emp_cv IS NULL THEN ...  -- illegal
      END;

      PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp) IS 
      BEGIN 
         emp_cv := NULL;  -- illegal
         ...
      END;

      DECLARE
         TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
         TYPE EmpCurTabTyp IS TABLE OF EmpCurTyp  -- illegal
            INDEX BY BINARY_INTEGER;

      DECLARE
         CURSOR emp_cur IS SELECT * FROM emp;  -- static cursor
         TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
         emp_cv  EmpCurTyp;  -- cursor variable
      BEGIN 
         ...
         FOR emp_rec IN emp_cv LOOP ...  -- illegal
            ...
         END LOOP;
      END;


Using Cursor Attributes

Each cursor or cursor variable has four attributes: %FOUND, %ISOPEN %NOTFOUND, and %ROWCOUNT. When appended to the cursor or cursor variable, these attributes return useful information about the execution of a data manipulation statement. You can use cursor attributes in procedural statements but not in SQL statements.

Explicit Cursor Attributes

Explicit cursor attributes return information about the execution of a multi-row query. When an explicit cursor or a cursor variable is opened, the rows that satisfy the associated query are identified and form the result set. Rows are fetched from the result set one at a time.

%FOUND

After a cursor or cursor variable is opened but before the first fetch, %FOUND yields NULL. Thereafter, it yields TRUE if the last fetch returned a row, or FALSE if the last fetch failed to return a row. In the following example, you use %FOUND to select an action:

LOOP
   FETCH c1 INTO my_ename, my_sal, my_hiredate;
   IF c1%FOUND THEN  -- fetch succeeded
      ...
   ELSE  -- fetch failed, so exit loop
      EXIT;
   END IF;
END LOOP;

If a cursor or cursor variable is not open, referencing it with %FOUND raises the predefined exception INVALID_CURSOR.

%ISOPEN

%ISOPEN yields TRUE if its cursor or cursor variable is open; otherwise, %ISOPEN yields FALSE. In the following example, you use %ISOPEN to select an action:

IF c1%ISOPEN THEN  -- cursor is open
   ...
ELSE  -- cursor is closed, so open it
   OPEN c1;
END IF;

%NOTFOUND

%NOTFOUND is the logical opposite of %FOUND. %NOTFOUND yields FALSE if the last fetch returned a row, or TRUE if the last fetch failed to return a row. In the following example, you use %NOTFOUND to exit a loop when FETCH fails to return a row:

LOOP
   FETCH c1 INTO my_ename, my_sal, my_hiredate;
   EXIT WHEN c1%NOTFOUND;
   ...
END LOOP;

If a cursor or cursor variable is not open, referencing it with %NOTFOUND raises INVALID_CURSOR.

%ROWCOUNT

When its cursor or cursor variable is opened, %ROWCOUNT is zeroed. Before the first fetch, %ROWCOUNT yields 0. Thereafter, it yields the number of rows fetched so far. The number is incremented if the last fetch returned a row. In the next example, you use %ROWCOUNT to take action if more than ten rows have been fetched:

LOOP
   FETCH c1 INTO my_ename, my_deptno;
   IF c1%ROWCOUNT > 10 THEN
      ...
   END IF;
   ...
END LOOP;

If a cursor or cursor variable is not open, referencing it with %ROWCOUNT raises INVALID_CURSOR.

Table 5 - 1 shows what each cursor attribute yields before and after you execute an OPEN, FETCH, or CLOSE statement.

%FOUND %ISOPEN %NOTFOUND %ROWCOUNT
OPEN before exception FALSE exception exception
after NULL TRUE NULL 0
first FETCH before NULL TRUE NULL 0
after TRUE TRUE FALSE 1
middle FETCHes before TRUE TRUE FALSE 1
after TRUE TRUE FALSE data dependent
last FETCH before TRUE TRUE FALSE data dependent
after FALSE TRUE TRUE data dependent
CLOSE before FALSE TRUE TRUE data dependent
after exception FALSE exception exception
Notes: 1. Referencing %FOUND, %NOTFOUND, or %ROWCOUNT before a cursor is opened or after it is closed raises INVALID_CURSOR. 2. After the first FETCH, if the result set was empty, %FOUND yields FALSE, %NOTFOUND yields TRUE, and %ROWCOUNT yields 0.
Table 5 - 1. Cursor Attribute Values

Some Examples

Suppose you have a table named data_table that holds data collected from laboratory experiments, and you want to analyze the data from experiment 1. In the following example, you compute the results and store them in a database table named temp:

-- available online in file EXAMP5
DECLARE
   num1   data_table.n1%TYPE;  -- Declare variables
   num2   data_table.n2%TYPE;  -- having same types as
   num3   data_table.n3%TYPE;  -- database columns
   result temp.col1%TYPE;
   CURSOR c1 IS
      SELECT n1, n2, n3 FROM data_table WHERE exper_num = 1;
BEGIN
   OPEN c1;
   LOOP
      FETCH c1 INTO num1, num2, num3;
      EXIT WHEN c1%NOTFOUND;  -- yields TRUE when FETCH
                              -- finds no more rows
      result := num2/(num1 + num3);
      INSERT INTO temp VALUES (result, NULL, NULL);
   END LOOP;
   CLOSE c1;
   COMMIT;
END;

In the next example, you check all storage bins that contain part number 5469, withdrawing their contents until you accumulate 1000 units:

-- available online in file EXAMP6
DECLARE
   CURSOR bin_cur(part_number NUMBER) IS 
      SELECT amt_in_bin FROM bins
         WHERE part_num = part_number AND amt_in_bin > 0
         ORDER BY bin_num
         FOR UPDATE OF amt_in_bin;
   bin_amt        bins.amt_in_bin%TYPE;
   total_so_far   NUMBER(5) := 0;
   amount_needed  CONSTANT NUMBER(5) := 1000;
   bins_looked_at NUMBER(3) := 0;
BEGIN
   OPEN bin_cur(5469);

   WHILE total_so_far < amount_needed LOOP
      FETCH bin_cur INTO bin_amt;
      EXIT WHEN bin_cur%NOTFOUND;
         -- if we exit, there's not enough to fill the order
      bins_looked_at := bins_looked_at + 1;
      IF total_so_far + bin_amt < amount_needed THEN
         UPDATE bins SET amt_in_bin = 0
             WHERE CURRENT OF bin_cur;
                -- take everything in the bin
         total_so_far := total_so_far + bin_amt;
      ELSE  -- we finally have enough
         UPDATE bins SET amt_in_bin = amt_in_bin
             - (amount_needed - total_so_far)
             WHERE CURRENT OF bin_cur;
         total_so_far := amount_needed;
      END IF;
   END LOOP;

   CLOSE bin_cur;

   INSERT INTO temp 
      VALUES (NULL, bins_looked_at, '<- bins looked at');
   COMMIT;
END;

Implicit Cursor Attributes

Implicit cursor attributes return information about the execution of an INSERT, UPDATE, DELETE, or SELECT INTO statement. The values of the cursor attributes always refer to the most recently executed SQL statement. Before Oracle opens the SQL cursor, the implicit cursor attributes yield NULL.

%FOUND

Until a SQL data manipulation statement is executed, %FOUND yields NULL. Thereafter, %FOUND yields TRUE if an INSERT, UPDATE, or DELETE statement affected one or more rows, or a SELECT INTO statement returned one or more rows. Otherwise, %FOUND yields FALSE. In the following example, you use %FOUND to insert a row if a delete succeeds:

DELETE FROM emp WHERE empno = my_empno;
IF SQL%FOUND THEN  -- delete succeeded
   INSERT INTO new_emp VALUES (my_empno, my_ename, ...);
   ...
END IF;

%ISOPEN

Oracle closes the SQL cursor automatically after executing its associated SQL statement. As a result, %ISOPEN always yields FALSE.

%NOTFOUND

%NOTFOUND is the logical opposite of %FOUND. %NOTFOUND yields TRUE if an INSERT, UPDATE, or DELETE statement affected no rows, or a SELECT INTO statement returned no rows. Otherwise, %NOTFOUND yields FALSE. In this example, you use %NOTFOUND to insert a new row if an update fails:

UPDATE emp SET sal = sal * 1.05 WHERE empno = my_empno;
IF SQL%NOTFOUND THEN  -- update failed
   INSERT INTO errors VALUES (...);
END IF;

%ROWCOUNT

%ROWCOUNT yields the number of rows affected by an INSERT, UPDATE, or DELETE statement, or returned by a SELECT INTO statement. %ROWCOUNT yields 0 if an INSERT, UPDATE, or DELETE statement affected no rows, or a SELECT INTO statement returned no rows. In the following example, you use %ROWCOUNT to take action if more than ten rows have been deleted:

DELETE FROM emp WHERE ...
IF SQL%ROWCOUNT > 10 THEN  -- more than 10 rows were deleted
   ...
END IF;

If a SELECT INTO statement returns more than one row, PL/SQL raises the predefined exception TOO_MANY_ROWS and %ROWCOUNT yields 1, not the actual number of rows that satisfy the query.

Guidelines

The values of the cursor attributes always refer to the most recently executed SQL statement, wherever that statement is. It might be in a different scope (for example, in a sub-block). So, if you want to save an attribute value for later use, assign it to a Boolean variable immediately. In the following example, relying on the IF condition is dangerous because the procedure check_status might have changed the value of %NOTFOUND:

UPDATE parts SET quantity = quantity - 1 WHERE partno = part_id;
check_status(part_id);  -- procedure call
IF SQL%NOTFOUND THEN  -- dangerous!

You can debug the code as follows:

UPDATE parts SET quantity = quantity - 1 WHERE partno = part_id;
sql_notfound := SQL%NOTFOUND;  -- assign value to Boolean variable
check_status(part_id);
IF sql_notfound THEN ...

If a SELECT INTO statement fails to return a row, PL/SQL raises the predefined exception NO_DATA_FOUND whether you check %NOTFOUND on the next line or not. Consider the following example:

BEGIN
   ...
   SELECT sal INTO my_sal FROM emp WHERE empno = my_empno;
      -- might raise NO_DATA_FOUND
   IF SQL%NOTFOUND THEN  -- condition tested only when false
      ...  -- this action is never taken
   END IF;

The check is useless because the IF condition is tested only when %NOTFOUND is false. When PL/SQL raises NO_DATA_FOUND, normal execution stops and control transfers to the exception-handling part of the block.

However, a SELECT INTO statement that calls a SQL group function never raises NO_DATA_FOUND because group functions always return a value or a null. In such cases, %NOTFOUND yields FALSE, as the following example shows:

BEGIN
   ...
   SELECT MAX(sal) INTO my_sal FROM emp WHERE deptno = my_deptno;
      -- never raises NO_DATA_FOUND
   IF SQL%NOTFOUND THEN  -- always tested but never true
      ...  -- this action is never taken
   END IF;
EXCEPTION
   WHEN NO_DATA_FOUND THEN ...  -- never invoked


Processing Transactions

This section explains how to do transaction processing. You learn the basic techniques that safeguard the consistency of your database, including how to control whether changes to Oracle data are made permanent or undone. Before delving into the subject, you should know the terms defined below.

The jobs or tasks that Oracle manages are called sessions. A user session is started when you run an application program or an Oracle tool and connect to Oracle. To allow user sessions to work "simultaneously" and share computer resources, Oracle must control concurrency, the accessing of the same data by many users. Without adequate concurrency controls, there might be a loss of data integrity. That is, changes to data or structures might be made in the wrong order.

Oracle uses locks to control concurrent access to data. A lock gives you temporary ownership of a database resource such as a table or row of data. Thus, data cannot be changed by other users until you finish with it. You need never explicitly lock a resource because default locking mechanisms protect Oracle data and structures. However, you can request data locks on tables or rows when it is to your advantage to override default locking. You can choose from several modes of locking such as row share and exclusive.

A deadlock can occur when two or more users try to access the same database object. For example, two users updating the same table might wait if each tries to update a row currently locked by the other. Because each user is waiting for resources held by another user, neither can continue until Oracle breaks the deadlock by signaling an error to the last participating transaction.

When a table is being queried by one user and updated by another at the same time, Oracle generates a read-consistent view of the data for the query. That is, once a query begins and as it proceeds, the data read by the query does not change. As update activity continues, Oracle takes snapshots of the table's data and records changes in a rollback segment. Oracle uses information in the rollback segment to build read-consistent query results and to undo changes if necessary.

How Transactions Guard Your Database

Oracle is transaction oriented; that is, it uses transactions to ensure data integrity. A transaction is a series of one or more logically related SQL statements that accomplish a task. Oracle treats the series of SQL statements as a unit so that all the changes brought about by the statements are either committed (made permanent) or rolled back (undone) at the same time. If your program fails in the middle of a transaction, the database is automatically restored to its former state.

The first SQL statement in your program begins a transaction. When one transaction ends, the next SQL statement automatically begins another transaction. Thus, every SQL statement is part of a transaction. A distributed transaction includes at least one SQL statement that updates data at multiple nodes in a distributed database.

The COMMIT and ROLLBACK statements ensure that all database changes brought about by SQL operations are either made permanent or undone at the same time. All the SQL statements executed since the last commit or rollback make up the current transaction. The SAVEPOINT statement names and marks the current point in the processing of a transaction.

Using COMMIT

The COMMIT statement ends the current transaction and makes permanent any changes made during that transaction. Until you commit the changes, other users cannot access the changed data; they see the data as it was before you made the changes.

Consider a simple transaction that transfers money from one bank account to another. The transaction requires two updates because it debits the first account, then credits the second. In the example below, after crediting the second account, you issue a commit, which makes the changes permanent. Only then do other users see the changes.

BEGIN
   ...
   UPDATE accts SET bal = my_bal - debit
      WHERE acctno = 7715;
   ...
   UPDATE accts SET bal = my_bal + credit
      WHERE acctno = 7720;
   COMMIT WORK;
END;

The COMMIT statement releases all row and table locks. It also erases any savepoints (discussed later) marked since the last commit or rollback. The optional keyword WORK has no effect other than to improve readability. The keyword END signals the end of a PL/SQL block, not the end of a transaction. Just as a block can span multiple transactions, a transaction can span multiple blocks.

The COMMENT clause lets you specify a comment to be associated with a distributed transaction. When you issue a commit, changes to each database affected by a distributed transaction are made permanent. However, if a network or machine fails during the commit, the state of the distributed transaction might be unknown or in doubt. In that case, Oracle stores the text specified by COMMENT in the data dictionary along with the transaction ID. The text must be a quoted literal up to 50 characters long. An example follows:

COMMIT COMMENT 'In-doubt order transaction; notify Order Entry';

PL/SQL does not support the FORCE clause, which, in SQL, manually commits an in-doubt distributed transaction. For example, the following COMMIT statement is illegal:

COMMIT FORCE '23.51.54';  -- illegal

Using ROLLBACK

The ROLLBACK statement ends the current transaction and undoes any changes made during that transaction. Rolling back is useful for two reasons. First, if you make a mistake like deleting the wrong row from a table, a rollback restores the original data. Second, if you start a transaction that you cannot finish because an exception is raised or a SQL statement fails, a rollback lets you return to the starting point to take corrective action and perhaps try again.

Consider the example below, in which you insert information about an employee into three different database tables. All three tables have a column that holds employee numbers and is constrained by a unique index. If an INSERT statement tries to store a duplicate employee number, the predefined exception DUP_VAL_ON_INDEX is raised. In that case, you want to undo all changes. So, you issue a rollback in the exception handler.

DECLARE
   emp_id  INTEGER;
   ...
BEGIN
   SELECT empno, ... INTO emp_id, ... FROM new_emp WHERE ...
   ...
   INSERT INTO emp VALUES (emp_id, ...);
   INSERT INTO tax VALUES (emp_id, ...);
   INSERT INTO pay VALUES (emp_id, ...);
   ...
EXCEPTION
   WHEN DUP_VAL_ON_INDEX THEN
      ROLLBACK;
   ...
END;

Statement-Level Rollbacks

Before executing a SQL statement, Oracle marks an implicit savepoint. Then, if the statement fails, Oracle rolls it back automatically. For example, if an INSERT statement raises an exception by trying to insert a duplicate value in a unique index, the statement is rolled back. Only work started by the failed SQL statement is lost. Work done before that statement in the current transaction is kept.

Oracle can also roll back single SQL statements to break deadlocks. Oracle signals an error to one of the participating transactions and rolls back the current statement in that transaction.

Before executing a SQL statement, Oracle must parse it, that is, examine it to make sure it follows syntax rules and refers to valid database objects. Errors detected while executing a SQL statement cause a rollback, but errors detected while parsing the statement do not.

Using SAVEPOINT

SAVEPOINT names and marks the current point in the processing of a transaction. Used with the ROLLBACK TO statement, savepoints let you undo parts of a transaction instead of the whole transaction. In the example below, you mark a savepoint before doing an insert. If the INSERT statement tries to store a duplicate value in the empno column, the predefined exception DUP_VAL_ON_INDEX is raised. In that case, you roll back to the savepoint, undoing just the insert.

DECLARE
   emp_id  emp.empno%TYPE;
BEGIN
   ...
   UPDATE emp SET ... WHERE empno = emp_id;
   DELETE FROM emp WHERE ...
   ...
   SAVEPOINT do_insert;
   INSERT INTO emp VALUES (emp_id, ...);
EXCEPTION
   WHEN DUP_VAL_ON_INDEX THEN
      ROLLBACK TO do_insert;
END;

When you roll back to a savepoint, any savepoints marked after that savepoint are erased. However, the savepoint to which you roll back is not erased. For example, if you mark five savepoints, then roll back to the third, only the fourth and fifth are erased. A simple rollback or commit erases all savepoints.

If you mark a savepoint within a recursive subprogram, new instances of the SAVEPOINT statement are executed at each level in the recursive descent. However, you can only rollback to the most recently marked savepoint.

Savepoint names are undeclared identifiers and can be reused within a transaction. This moves the savepoint from its old position to the current point in the transaction. Thus, a rollback to the savepoint affects only the current part of your transaction. An example follows:

BEGIN
   ...
   SAVEPOINT my_point;
   UPDATE emp SET ... WHERE empno = emp_id;
   ...
   SAVEPOINT my_point;  -- move my_point to current point
   INSERT INTO emp VALUES (emp_id, ...);
   ...
EXCEPTION
   WHEN OTHERS THEN
      ROLLBACK TO my_point;
END;

By default, the number of active savepoints per session is limited to five. An active savepoint is one marked since the last commit or rollback. You or your DBA can raise the limit (up to 255) by increasing the value of the Oracle initialization parameter SAVEPOINTS.

Implicit Rollbacks

Before executing an INSERT, UPDATE, or DELETE statement, Oracle marks an implicit savepoint (unavailable to you). If the statement fails, Oracle rolls back to the savepoint. Normally, just the failed SQL statement is rolled back, not the whole transaction. However, if the statement raises an unhandled exception, the host environment determines what is rolled back.

If you exit a stored subprogram with an unhandled exception, PL/SQL does not assign values to OUT parameters. Also, PL/SQL does not roll back database work done by the subprogram.

Ending Transactions

It is good programming practice to commit or roll back every transaction explicitly. Whether you issue the commit or rollback in your PL/SQL program or in the host environment depends on the flow of application logic. If you neglect to commit or roll back a transaction explicitly, the host environment determines its final state.

For example, in the SQL*Plus environment, if your PL/SQL block does not include a COMMIT or ROLLBACK statement, the final state of your transaction depends on what you do after running the block. If you execute a data definition, data control, or COMMIT statement or if you issue the EXIT, DISCONNECT, or QUIT command, Oracle commits the transaction. If you execute a ROLLBACK statement or abort the SQL*Plus session, Oracle rolls back the transaction.

In the Oracle Precompiler environment, if your program does not terminate normally, Oracle rolls back your transaction. A program terminates normally when it explicitly commits or rolls back work and disconnects from Oracle using the RELEASE parameter, as follows:

EXEC SQL COMMIT WORK RELEASE;

In the OCI environment, if you issue the OLOGOF call, Oracle automatically commits your transaction. Otherwise, Oracle rolls back the transaction.

Using SET TRANSACTION

You use the SET TRANSACTION statement to begin a read-only or read-write transaction, establish an isolation level, or assign your current transaction to a specified rollback segment. Read-only transactions are useful for running multiple queries against one or more tables while other users update the same tables.

During a read-only transaction, all queries refer to the same snapshot of the database, providing a multi-table, multi-query, read-consistent view. Other users can continue to query or update data as usual. A commit or rollback ends the transaction. In the example below, as a store manager, you use a read-only transaction to gather sales figures for the day, the past week, and the past month. The figures are unaffected by other users updating the database during the transaction.

DECLARE
   daily_sales   REAL;
   weekly_sales  REAL;
   monthly_sales REAL;
BEGIN
   ...
   COMMIT;  -- ends previous transaction
   SET TRANSACTION READ ONLY;
   SELECT SUM(amt) INTO daily_sales FROM sales
      WHERE dte = SYSDATE;
   SELECT SUM(amt) INTO weekly_sales FROM sales
      WHERE dte > SYSDATE - 7;
   SELECT SUM(amt) INTO monthly_sales FROM sales
      WHERE dte > SYSDATE - 30;
   COMMIT;  -- ends read-only transaction
   ...
END;

The SET TRANSACTION statement must be the first SQL statement in a read-only transaction and can only appear once in a transaction. If you set a transaction to READ ONLY, subsequent queries see only changes committed before the transaction began. The use of READ ONLY does not affect other users or transactions.

Restrictions

Only the SELECT INTO, OPEN, FETCH, CLOSE, LOCK TABLE, COMMIT, and ROLLBACK statements are allowed in a read-only transaction. Also, queries cannot be FOR UPDATE.

Overriding Default Locking

By default, Oracle locks data structures for you automatically. However, you can request specific data locks on rows or tables when it is to your advantage to override default locking. Explicit locking lets you share or deny access to a table for the duration of a transaction.

With the SELECT FOR UPDATE statement, you can explicitly lock specific rows of a table to make sure they do not change before an update or delete is executed. However, Oracle automatically obtains row-level locks at update or delete time. So, use the FOR UPDATE clause only if you want to lock the rows before the update or delete.

You can explicitly lock entire tables using the LOCK TABLE statement.

Using FOR UPDATE

When you declare a cursor that will be referenced in the CURRENT OF clause of an UPDATE or DELETE statement, you must use the FOR UPDATE clause to acquire exclusive row locks. An example follows:

DECLARE
   CURSOR c1 IS SELECT empno, sal FROM emp
      WHERE job = 'SALESMAN' AND comm > sal FOR UPDATE NOWAIT;

The FOR UPDATE clause identifies the rows that will be updated or deleted, then locks each row in the result set. This is useful when you want to base an update on the existing values in a row. In that case, you must make sure the row is not changed by another user before the update.

The optional keyword NOWAIT tells Oracle not to wait if the table has been locked by another user. Control is immediately returned to your program so that it can do other work before trying again to acquire the lock. If you omit the keyword NOWAIT, Oracle waits until the table is available. The wait has no limit unless the table is remote, in which case the Oracle initialization parameter DISTRIBUTED_LOCK_TIMEOUT sets a limit.

All rows are locked when you open the cursor, not as they are fetched. The rows are unlocked when you commit or roll back the transaction. So, you cannot fetch from a FOR UPDATE cursor after a commit. (For a workaround, see "Fetching Across Commits" [*].)

When querying multiple tables, you can use the FOR UPDATE clause to confine row locking to particular tables. Rows in a table are locked only if the FOR UPDATE OF clause refers to a column in that table. For example, the following query locks rows in the emp table but not in the dept table:

DECLARE
   CURSOR c1 IS SELECT ename, dname FROM emp, dept
      WHERE emp.deptno = dept.deptno AND job = 'MANAGER'
      FOR UPDATE OF sal;

You use the CURRENT OF clause in an UPDATE or DELETE statement to refer to the latest row fetched from a cursor, as the following example shows:

DECLARE
   CURSOR c1 IS SELECT empno, job, sal FROM emp FOR UPDATE;
   ...
BEGIN
   OPEN c1;
   LOOP
      FETCH c1 INTO ...
      ...
      UPDATE emp SET sal = new_sal WHERE CURRENT OF c1;
   END LOOP;

Using LOCK TABLE

You use the LOCK TABLE statement to lock entire database tables in a specified lock mode so that you can share or deny access to them. For example, the statement below locks the emp table in row share mode. Row share locks allow concurrent access to a table; they prevent other users from locking the entire table for exclusive use. Table locks are released when your transaction issues a commit or rollback.

LOCK TABLE emp IN ROW SHARE MODE NOWAIT;

The lock mode determines what other locks can be placed on the table. For example, many users can acquire row share locks on a table at the same time, but only one user at a time can acquire an exclusive lock. While one user has an exclusive lock on a table, no other users can insert, delete, or update rows in that table. For more information about lock modes, see Oracle7 Server Application Developer's Guide.

A table lock never keeps other users from querying a table, and a query never acquires a table lock. Only if two different transactions try to modify the same row will one transaction wait for the other to complete.

Note: If your program includes SQL locking statements, make sure the Oracle users requesting locks have the privileges needed to obtain the locks.

Fetching Across Commits

Remember, the FOR UPDATE clause acquires exclusive row locks. All rows are locked when you open the cursor, and they are unlocked when you commit your transaction. So, you cannot fetch from a FOR UPDATE cursor after a commit. If you do, PL/SQL raises an exception. In the following example, the cursor FOR loop fails after the tenth insert:

DECLARE
   CURSOR c1 IS SELECT ename FROM emp FOR UPDATE OF sal;
   ctr NUMBER := 0;
BEGIN
   FOR emp_rec IN c1 LOOP  -- FETCHes implicitly
      ...
      ctr := ctr + 1;
      INSERT INTO temp VALUES (ctr, 'still going');
      IF ctr >= 10 THEN
         COMMIT;  -- releases locks
      END IF;
   END LOOP;
END;

If you want to fetch across commits, do not use the FOR UPDATE and CURRENT OF clauses. Instead, use the ROWID pseudocolumn to mimic the CURRENT OF clause. Simply select the rowid of each row into a ROWID variable. Then, use the rowid to identify the current row during subsequent updates and deletes. An example follows:

DECLARE
   CURSOR c1 IS SELECT ename, job, rowid FROM emp;
   my_ename  emp.ename%TYPE;
   my_job    emp.job%TYPE;
   my_rowid  ROWID;
BEGIN
   OPEN c1;
   LOOP
      FETCH c1 INTO my_ename, my_job, my_rowid;
      EXIT WHEN c1%NOTFOUND;
      UPDATE emp SET sal = sal * 1.05 WHERE rowid = my_rowid;
      -- this mimics WHERE CURRENT OF c1
      COMMIT;
   END LOOP;
   CLOSE c1;
END;

Be careful. In the last example, the fetched rows are not locked because no FOR UPDATE clause is used. So, other users might unintentionally overwrite your changes. Also, the cursor must have a read-consistent view of the data, so rollback segments used in the update are not released until the cursor is closed. This can slow down processing when many rows are updated.

The next example shows that you can use the %ROWTYPE attribute with cursors that reference the ROWID pseudocolumn:

DECLARE
   CURSOR c1 IS SELECT ename, sal, rowid FROM emp;
   emp_rec c1%ROWTYPE;
BEGIN
   OPEN c1;
   LOOP
      FETCH c1 INTO emp_rec;
      EXIT WHEN c1%NOTFOUND;
      ...
      IF ... THEN
         DELETE FROM emp WHERE rowid = emp_rec.rowid;
      END IF;
   END LOOP;
   CLOSE c1;
END;

Dealing with Size Limitations

PL/SQL was designed primarily for transaction processing. As a result, the PL/SQL compiler limits the number of tokens a block can generate. Blocks that exceed the limit cause a program too large compilation error. Generally, blocks larger than 64K exceed the token limit. However, much smaller blocks can exceed the limit if they contain many variables or complex SQL statements.

The best solution to this problem is to modularize your program by defining subprograms (which can be stored in an Oracle database). For more information, see Chapter 7.

Another solution is to break the block into two sub-blocks. Before the first block terminates, have it insert any data the second block needs into a database table called temp (for example). When the second block starts executing, have it select the data from temp. This approximates the passing of parameters from one procedure to another. The following example shows two "parameter passing" PL/SQL blocks in a SQL*Plus script:

DECLARE
   mode   NUMBER;
   median NUMBER;
BEGIN
   ...
   INSERT INTO temp (col1, col2, col3)
      VALUES (mode, median, 'blockA');
END;
/
...
DECLARE
   mode   NUMBER;
   median NUMBER;
BEGIN
   SELECT col1, col2 INTO mode, median FROM temp
      WHERE col3 = 'blockA';
   ...
END;
/

The previous method works unless you must re-execute the first block while the second block is still executing or unless two or more users must run the script concurrently. To avoid these restrictions, embed your PL/SQL blocks in a third-generation host language such as C, COBOL, or FORTRAN. That way, you can re-execute the first block using flow-of-control statements. Also, you can store data in global host variables instead of using a temporary database table. In the following example, you embed two PL/SQL blocks in a Pro*C program:

EXEC SQL BEGIN DECLARE SECTION;
   int   my_empno;
   float my_sal, my_comm;
   short comm_ind;
   ...
EXEC SQL END DECLARE SECTION;
...
EXEC SQL EXECUTE
   BEGIN
      ...
      SELECT sal, comm INTO :my_sal, :my_comm:comm_ind FROM emp
         WHERE empno = :my_empno;
      IF :my_comm:comm_ind IS NULL THEN
         ...
      END IF;
   END;
END-EXEC;
...
EXEC SQL EXECUTE
   BEGIN
      ...
      IF :my_comm:comm_ind > 1000 THEN
         :my_sal := :my_sal * 1.10;
         UPDATE emp SET sal = :my_sal WHERE empno = :my_empno;
      END IF;
   END;
END-EXEC;
...




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