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7 Performing SQL Operations with Native Dynamic SQL

A happy and gracious flexibility ... — Matthew Arnold

This chapter shows you how to use native dynamic SQL (dynamic SQL for short), a PL/SQL interface that makes your programs more flexible, by building and processing SQL statements at run time.

With dynamic SQL, you can directly execute any kind of SQL statement (even data definition and data control statements). You can build statements where you do not know table names, WHERE clauses, and other information in advance.

This chapter contains these topics:

What Is Dynamic SQL?

Some programs must build and process SQL statements where some information is not known in advance. A reporting application might build different SELECT statements for the various reports it generates, substituting new table and column names and ordering or grouping by different columns. Database management applications might issue statements such as CREATE, DROP, and GRANT that cannot be coded directly in a PL/SQL program. These statements are called dynamic SQL statements.

Dynamic SQL statements built as character strings built at run time. The strings contain the text of a SQL statement or PL/SQL block. They can also contain placeholders for bind arguments. Placeholder names are prefixed by a colon, and the names themselves do not matter. For example, PL/SQL makes no distinction between the following strings:

'DELETE FROM emp WHERE sal > :my_sal AND comm < :my_comm'
'DELETE FROM emp WHERE sal > :s AND comm < :c'

To process most dynamic SQL statements, you use the EXECUTE IMMEDIATE statement. To process a multi-row query (SELECT statement), you use the OPEN-FOR, FETCH, and CLOSE statements.

Why Use Dynamic SQL?

You need dynamic SQL in the following situations:

If you have older code that uses the DBMS_SQL package, the techniques described in this chapter using EXECUTE IMMEDIATE and OPEN-FOR generally provide better performance, more readable code, and extra features such as support for objects and collections. (For a comparison with DBMS_SQL, see Oracle Database Application Developer's Guide - Fundamentals.)

Using the EXECUTE IMMEDIATE Statement

The EXECUTE IMMEDIATE statement prepares (parses) and immediately executes a dynamic SQL statement or an anonymous PL/SQL block.

The main argument to EXECUTE IMMEDIATE is the string containing the SQL statement to execute. You can build up the string using concatenation, or use a predefined string.

Except for multi-row queries, the dynamic string can contain any SQL statement (without the final semicolon) or any PL/SQL block (with the final semicolon). The string can also contain placeholders, arbitrary names preceded by a colon, for bind arguments. In this case, you specify which PL/SQL variables correspond to the placeholders with the INTO, USING, and RETURNING INTO clauses.

You can only use placeholders in places where you can substitute variables in the SQL statement, such as conditional tests in WHERE clauses. You cannot use placeholders for the names of schema objects. For the right way, see "Passing Schema Object Names As Parameters".

Used only for single-row queries, the INTO clause specifies the variables or record into which column values are retrieved. For each value retrieved by the query, there must be a corresponding, type-compatible variable or field in the INTO clause.

Used only for DML statements that have a RETURNING clause (without a BULK COLLECT clause), the RETURNING INTO clause specifies the variables into which column values are returned. For each value returned by the DML statement, there must be a corresponding, type-compatible variable in the RETURNING INTO clause.

You can place all bind arguments in the USING clause. The default parameter mode is IN. For DML statements that have a RETURNING clause, you can place OUT arguments in the RETURNING INTO clause without specifying the parameter mode. If you use both the USING clause and the RETURNING INTO clause, the USING clause can contain only IN arguments.

At run time, bind arguments replace corresponding placeholders in the dynamic string. Every placeholder must be associated with a bind argument in the USING clause and/or RETURNING INTO clause. You can use numeric, character, and string literals as bind arguments, but you cannot use Boolean literals (TRUE, FALSE, and NULL). To pass nulls to the dynamic string, you must use a workaround. See "Passing Nulls to Dynamic SQL".

Dynamic SQL supports all the SQL datatypes. For example, define variables and bind arguments can be collections, LOBs, instances of an object type, and refs.

As a rule, dynamic SQL does not support PL/SQL-specific types. For example, define variables and bind arguments cannot be Booleans or associative arrays. The only exception is that a PL/SQL record can appear in the INTO clause.

You can execute a dynamic SQL statement repeatedly using new values for the bind arguments. However, you incur some overhead because EXECUTE IMMEDIATE re-prepares the dynamic string before every execution.

Example 7-1 Some Examples of Dynamic SQL

The following PL/SQL block contains several examples of dynamic SQL:

DECLARE
   sql_stmt    VARCHAR2(200);
   plsql_block VARCHAR2(500);
   emp_id      NUMBER(4) := 7566;
   salary      NUMBER(7,2);
   dept_id     NUMBER(2) := 50;
   dept_name   VARCHAR2(14) := 'PERSONNEL';
   location    VARCHAR2(13) := 'DALLAS';
   emp_rec     emp%ROWTYPE;
BEGIN
   EXECUTE IMMEDIATE 'CREATE TABLE bonus (id NUMBER, amt NUMBER)';
   sql_stmt := 'INSERT INTO dept VALUES (:1, :2, :3)';
   EXECUTE IMMEDIATE sql_stmt USING dept_id, dept_name, location;
   sql_stmt := 'SELECT * FROM emp WHERE empno = :id';
   EXECUTE IMMEDIATE sql_stmt INTO emp_rec USING emp_id;
   plsql_block := 'BEGIN emp_pkg.raise_salary(:id, :amt); END;';
   EXECUTE IMMEDIATE plsql_block USING 7788, 500;
   sql_stmt := 'UPDATE emp SET sal = 2000 WHERE empno = :1
      RETURNING sal INTO :2';
   EXECUTE IMMEDIATE sql_stmt USING emp_id RETURNING INTO salary;
   EXECUTE IMMEDIATE 'DELETE FROM dept WHERE deptno = :num'
      USING dept_id;
   EXECUTE IMMEDIATE 'ALTER SESSION SET SQL_TRACE TRUE';
END;
/

Example 7-2 Dynamic SQL Procedure that Accepts Table Name and WHERE Clause

In the example below, a standalone procedure accepts the name of a database table and an optional WHERE-clause condition. If you omit the condition, the procedure deletes all rows from the table. Otherwise, the procedure deletes only those rows that meet the condition.

CREATE OR REPLACE PROCEDURE delete_rows (
   table_name IN VARCHAR2,
   condition IN VARCHAR2 DEFAULT NULL) AS
   where_clause VARCHAR2(100) := ' WHERE ' || condition;
BEGIN
   IF condition IS NULL THEN where_clause := NULL; END IF;
   EXECUTE IMMEDIATE 'DELETE FROM ' || table_name || where_clause;
END;
/

Specifying Parameter Modes for Bind Variables in Dynamic SQL Strings

With the USING clause, the mode defaults to IN, so you do not need to specify a parameter mode for input bind arguments.

With the RETURNING INTO clause, the mode is OUT, so you cannot specify a parameter mode for output bind arguments.

You must specify the parameter mode in more complicated cases, such as this one where you call a procedure from a dynamic PL/SQL block:

CREATE PROCEDURE create_dept (
   deptno IN OUT NUMBER,
   dname  IN VARCHAR2,
   loc    IN VARCHAR2) AS
BEGIN
   SELECT deptno_seq.NEXTVAL INTO deptno FROM dual;
   INSERT INTO dept VALUES (deptno, dname, loc);
END;
/

To call the procedure from a dynamic PL/SQL block, you must specify the IN OUT mode for the bind argument associated with formal parameter deptno, as follows:

DECLARE
   plsql_block VARCHAR2(500);
   new_deptno NUMBER(2);
   new_dname  VARCHAR2(14) := 'ADVERTISING';
   new_loc    VARCHAR2(13) := 'NEW YORK';
BEGIN
   plsql_block := 'BEGIN create_dept(:a, :b, :c); END;';
   EXECUTE IMMEDIATE plsql_block
      USING IN OUT new_deptno, new_dname, new_loc;
   IF new_deptno > 90 THEN ...
END;
/

Building a Dynamic Query with Dynamic SQL

You use three statements to process a dynamic multi-row query: 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. (For more information about cursor variables, see "Using Cursor Variables (REF CURSORs)".)

Examples of Dynamic SQL for Records, Objects, and Collections

Example 7-3 Dynamic SQL Fetching into a Record

As the following example shows, you can fetch rows from the result set of a dynamic multi-row query into a record:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR;
   emp_cv   EmpCurTyp;
   emp_rec  emp%ROWTYPE;
   sql_stmt VARCHAR2(200);
   my_job   VARCHAR2(15) := 'CLERK';
BEGIN
   sql_stmt := 'SELECT * FROM emp WHERE job = :j';
   OPEN emp_cv FOR sql_stmt USING my_job;
   LOOP
      FETCH emp_cv INTO emp_rec;
      EXIT WHEN emp_cv%NOTFOUND;
      -- process record
   END LOOP;
   CLOSE emp_cv;
END;
/

Example 7-4 Dynamic SQL for Object Types and Collections

The next example illustrates the use of objects and collections. Suppose you define object type Person and VARRAY type Hobbies, as follows:

CREATE TYPE Person AS OBJECT (name VARCHAR2(25), age NUMBER);
CREATE TYPE Hobbies IS VARRAY(10) OF VARCHAR2(25);

Using dynamic SQL, you can write a package that uses these types:

CREATE OR REPLACE PACKAGE teams AS
   PROCEDURE create_table (tab_name VARCHAR2);
   PROCEDURE insert_row (tab_name VARCHAR2, p Person, h Hobbies);
   PROCEDURE print_table (tab_name VARCHAR2);
END;
/

CREATE OR REPLACE PACKAGE BODY teams AS
   PROCEDURE create_table (tab_name VARCHAR2) IS
   BEGIN
      EXECUTE IMMEDIATE 'CREATE TABLE ' || tab_name ||
         ' (pers Person, hobbs Hobbies)';
   END;

   PROCEDURE insert_row (
      tab_name VARCHAR2,
      p Person,
      h Hobbies) IS
   BEGIN
      EXECUTE IMMEDIATE 'INSERT INTO ' || tab_name ||
         ' VALUES (:1, :2)' USING p, h;
   END;

   PROCEDURE print_table (tab_name VARCHAR2) IS
      TYPE RefCurTyp IS REF CURSOR;
      cv RefCurTyp;
      p  Person;
      h  Hobbies;
   BEGIN
      OPEN cv FOR 'SELECT pers, hobbs FROM ' || tab_name;
      LOOP
         FETCH cv INTO p, h;
         EXIT WHEN cv%NOTFOUND;
         -- print attributes of 'p' and elements of 'h'
      END LOOP;
      CLOSE cv;
   END;
END;
/

From an anonymous block, you might call the procedures in package TEAMS:

DECLARE
   team_name VARCHAR2(15);
BEGIN
   team_name := 'Notables';
   teams.create_table(team_name);
   teams.insert_row(team_name, Person('John', 31),
      Hobbies('skiing', 'coin collecting', 'tennis'));
   teams.insert_row(team_name, Person('Mary', 28),
      Hobbies('golf', 'quilting', 'rock climbing'));
   teams.print_table(team_name);
END;
/

Using Bulk Dynamic SQL

Bulk SQL passes entire collections back and forth, not just individual elements. This technique improves performance by minimizing the number of context switches between the PL/SQL and SQL engines. You can use a single statement instead of a loop that issues a SQL statement in every iteration.

Using the following commands, clauses, and cursor attribute, your applications can construct bulk SQL statements, then execute them dynamically at run time:

BULK FETCH statement
BULK EXECUTE IMMEDIATE statement
FORALL statement
COLLECT INTO clause
RETURNING INTO clause
%BULK_ROWCOUNT cursor attribute

The static versions of these statements, clauses, and cursor attribute are discussed in "Reducing Loop Overhead for DML Statements and Queries (FORALL, BULK COLLECT)". Refer to that section for background information.

Using Dynamic SQL with Bulk SQL

Bulk binding lets Oracle bind a variable in a SQL statement to a collection of values. The collection type can be any PL/SQL collection type (index-by table, nested table, or varray). The collection elements must have a SQL datatype such as CHAR, DATE, or NUMBER. Three statements support dynamic bulk binds: EXECUTE IMMEDIATE, FETCH, and FORALL.


EXECUTE IMMEDIATE

You can use the BULK COLLECT INTO clause with the EXECUTE IMMEDIATE statement to store values from each column of a query's result set in a separate collection.

You can use the RETURNING BULK COLLECT INTO clause with the EXECUTE IMMEDIATE statement to store the results of an INSERT, UPDATE, or DELETE statement in a set of collections.


FETCH

You can use the BULK COLLECT INTO clause with the FETCH statement to store values from each column of a cursor in a separate collection.


FORALL

You can put an EXECUTE IMMEDIATE statement with the RETURNING BULK COLLECT INTO inside a FORALL statement. You can store the results of all the INSERT, UPDATE, or DELETE statements in a set of collections.

You can pass subscripted collection elements to the EXECUTE IMMEDIATE statement through the USING clause. You cannot concatenate the subscripted elements directly into the string argument to EXECUTE IMMEDIATE; for example, you cannot build a collection of table names and write a FORALL statement where each iteration applies to a different table.

Examples of Dynamic Bulk Binds

Example 7-5 Dynamic SQL with BULK COLLECT INTO Clause

You can bind define variables in a dynamic query using the BULK COLLECT INTO clause. As the following example shows, you can use that clause in a bulk FETCH or bulk EXECUTE IMMEDIATE statement:

DECLARE
   TYPE EmpCurTyp IS REF CURSOR;
   TYPE NumList IS TABLE OF NUMBER;
   TYPE NameList IS TABLE OF VARCHAR2(15);
   emp_cv EmpCurTyp;
   empnos NumList;
   enames NameList;
   sals   NumList;
BEGIN
   OPEN emp_cv FOR 'SELECT empno, ename FROM emp';
   FETCH emp_cv BULK COLLECT INTO empnos, enames;
   CLOSE emp_cv;

   EXECUTE IMMEDIATE 'SELECT sal FROM emp'
      BULK COLLECT INTO sals;
END;
/

Example 7-6 Dynamic SQL with RETURNING BULK COLLECT INTO Clause

Only INSERT, UPDATE, and DELETE statements can have output bind variables. You bulk-bind them with the RETURNING BULK COLLECT INTO clause of EXECUTE IMMEDIATE:

DECLARE
   TYPE NameList IS TABLE OF VARCHAR2(15);
   enames    NameList;
   bonus_amt NUMBER := 500;
   sql_stmt  VARCHAR(200);
BEGIN
   sql_stmt := 'UPDATE emp SET bonus = :1 RETURNING ename INTO :2';
   EXECUTE IMMEDIATE sql_stmt
      USING bonus_amt RETURNING BULK COLLECT INTO enames;
END;
/

Example 7-7 Dynamic SQL Inside FORALL Statement

To bind the input variables in a SQL statement, you can use the FORALL statement and USING clause, as shown below. The SQL statement cannot be a query.

DECLARE
   TYPE NumList IS TABLE OF NUMBER;
   TYPE NameList IS TABLE OF VARCHAR2(15);
   empnos NumList;
   enames NameList;
BEGIN
   empnos := NumList(1,2,3,4,5);
   FORALL i IN 1..5
      EXECUTE IMMEDIATE
        'UPDATE emp SET sal = sal * 1.1 WHERE empno = :1
         RETURNING ename INTO :2'
         USING empnos(i) RETURNING BULK COLLECT INTO enames;
   ...
END;
/

Guidelines for Dynamic SQL

This section shows you how to take full advantage of dynamic SQL and how to avoid some common pitfalls.

When to Use or Omit the Semicolon with Dynamic SQL

When building up a single SQL statement in a string, do not include any semicolon at the end.

When building up a PL/SQL anonymous block, include the semicolon at the end of each PL/SQL statement and at the end of the anonymous block.

For example:

BEGIN
   EXECUTE IMMEDIATE 'dbms_output.put_line(''No semicolon'')';
   EXECUTE IMMEDIATE 'BEGIN dbms_output.put_line(''semicolons''); END;';
END;

Improving Performance of Dynamic SQL with Bind Variables

When you code INSERT, UPDATE, DELETE, and SELECT statements directly in PL/SQL, PL/SQL turns the variables into bind variables automatically, to make the statements work efficiently with SQL. When you build up such statements in dynamic SQL, you need to specify the bind variables yourself to get the same performance.

In the example below, Oracle opens a different cursor for each distinct value of emp_id. This can lead to resource contention and poor performance as each statement is parsed and cached.

CREATE PROCEDURE fire_employee (emp_id NUMBER) AS
BEGIN
   EXECUTE IMMEDIATE
      'DELETE FROM emp WHERE empno = ' || TO_CHAR(emp_id);
END;
/

You can improve performance by using a bind variable, which allows Oracle to reuse the same cursor for different values of emp_id:

CREATE PROCEDURE fire_employee (emp_id NUMBER) AS
BEGIN
   EXECUTE IMMEDIATE
      'DELETE FROM emp WHERE empno = :num' USING emp_id;
END;
/

Passing Schema Object Names As Parameters

Suppose you need a procedure that accepts the name of any database table, then drops that table from your schema. You must build a string with a statement that includes the object names, then use EXECUTE IMMEDIATE to execute the statement:

CREATE PROCEDURE drop_table (table_name IN VARCHAR2) AS
BEGIN
   EXECUTE IMMEDIATE 'DROP TABLE ' || table_name;
END;
/

Use concatenation to build the string, rather than trying to pass the table name as a bind variable through the USING clause.

Using Duplicate Placeholders with Dynamic SQL

Placeholders in a dynamic SQL statement are associated with bind arguments in the USING clause by position, not by name. If you specify a sequence of placeholders like :a, :a, :b, :b, you must include four items in the USING clause. For example, given the dynamic string

sql_stmt := 'INSERT INTO payroll VALUES (:x, :x, :y, :x)';

the fact that the name X is repeated is not significant. You can code the corresponding USING clause with four different bind variables:

EXECUTE IMMEDIATE sql_stmt USING a, a, b, a;

If the dynamic statement represents a PL/SQL block, the rules for duplicate placeholders are different. Each unique placeholder maps to a single item in the USING clause. If the same placeholder appears two or more times, all references to that name correspond to one bind argument in the USING clause. In the following example, all references to the placeholder X are associated with the first bind argument A, and the second unique placeholder Y is associated with the second bind argument B.

DECLARE
   a NUMBER := 4;
   b NUMBER := 7;
BEGIN
   plsql_block := 'BEGIN calc_stats(:x, :x, :y, :x); END;'
   EXECUTE IMMEDIATE plsql_block USING a, b;
END;
/

Using Cursor Attributes with Dynamic SQL

The SQL cursor attributes %FOUND, %ISOPEN, %NOTFOUND, and %ROWCOUNT work when you issue an INSERT, UPDATE, DELETE, or single-row SELECT statement in dynamic SQL:

EXECUTE IMMEDIATE 'DELETE FROM employees WHERE employee_id > 1000';
rows_deleted := SQL%ROWCOUNT;

Likewise, when appended to a cursor variable name, the cursor attributes return information about the execution of a multi-row query:

OPEN c1 FOR 'SELECT * FROM employees';
FETCH c1 BULK COLLECT INTO rec_tab;
rows_fetched := c1%ROWCOUNT;

For more information about cursor attributes, see "Using Cursor Expressions".

Passing Nulls to Dynamic SQL

The literal NULL is not allowed in the USING clause. To work around this restriction, replace the keyword NULL with an uninitialized variable:

DECLARE
   a_null CHAR(1); -- set to NULL automatically at run time
BEGIN
   EXECUTE IMMEDIATE 'UPDATE emp SET comm = :x' USING a_null;
END;
/

Using Database Links with Dynamic SQL

PL/SQL subprograms can execute dynamic SQL statements that use database links to refer to objects on remote databases:

PROCEDURE delete_dept (db_link VARCHAR2, dept_id INTEGER) IS
BEGIN
   EXECUTE IMMEDIATE 'DELETE FROM departments@' || db_link ||
      ' WHERE deptno = :num' USING dept_id;
END;
/

The targets of remote procedure calls (RPCs) can contain dynamic SQL statements. For example, suppose the following standalone function, which returns the number of rows in a table, resides on the Chicago database:

CREATE FUNCTION row_count (tab_name VARCHAR2) RETURN INTEGER AS
   rows INTEGER;
BEGIN
   EXECUTE IMMEDIATE 'SELECT COUNT(*) FROM ' || tab_name INTO rows;
   RETURN rows;
END;
/

From an anonymous block, you might call the function remotely, as follows:

DECLARE
   emp_count INTEGER;
BEGIN
   emp_count := row_count@chicago('employees');
END;
/

Using Invoker Rights with Dynamic SQL

Dynamic SQL lets you write schema-management procedures that can be centralized in one schema, and can be called from other schemas and operate on the objects in those schemas.

For example, this procedure can drop any kind of database object:

CREATE OR REPLACE PROCEDURE drop_it (kind IN VARCHAR2, name IN
VARCHAR2)
AUTHID CURRENT_USER
AS
BEGIN
   EXECUTE IMMEDIATE 'DROP ' || kind || ' ' || name;
END;
/

Let's say that this procedure is part of the HR schema. Without the AUTHID clause, the procedure would always drop objects in the HR schema, regardless of who calls it. Even if you pass a fully qualified object name, this procedure would not have the privileges to make changes in other schemas.

The AUTHID clause lifts both of these restrictions. It lets the procedure run with the privileges of the user that invokes it, and makes unqualified references refer to objects in that user's schema.

For details, see "Using Invoker's Rights Versus Definer's Rights (AUTHID Clause)".

Using Pragma RESTRICT_REFERENCES with Dynamic SQL

A function called from SQL statements must obey certain rules meant to control side effects. (See "Controlling Side Effects of PL/SQL Subprograms".) To check for violations of the rules, you can use the pragma RESTRICT_REFERENCES. The pragma asserts that a function does not read or write database tables or package variables. (For more information, See Oracle Database Application Developer's Guide - Fundamentals.)

If the function body contains a dynamic INSERT, UPDATE, or DELETE statement, the function always violates the rules "write no database state" (WNDS) and "read no database state" (RNDS). PL/SQL cannot detect those side-effects automatically, because dynamic SQL statements are checked at run time, not at compile time. In an EXECUTE IMMEDIATE statement, only the INTO clause can be checked at compile time for violations of RNDS.

Avoiding Deadlocks with Dynamic SQL

In a few situations, executing a SQL data definition statement results in a deadlock. For example, the procedure below causes a deadlock because it attempts to drop itself. To avoid deadlocks, never try to ALTER or DROP a subprogram or package while you are still using it.

CREATE OR REPLACE PROCEDURE calc_bonus (emp_id NUMBER) AS
BEGIN
   EXECUTE IMMEDIATE 'DROP PROCEDURE calc_bonus'; -- deadlock!
END;
/

Backward Compatibility of the USING Clause

When a dynamic INSERT, UPDATE, or DELETE statement has a RETURNING clause, output bind arguments can go in the RETURNING INTO clause or the USING clause. In new applications, use the RETURNING INTO clause. In old applications, you can continue to use the USING clause.