PL/SQL Language Elements, 13 of 52

Cursor Variables

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. To create cursor variables, you define a REF CURSOR type, then declare cursor variables of that type.

Cursor variables are like C or Pascal pointers, which hold the memory location (address) of some item instead of the item itself. So, declaring a cursor variable creates a pointer, not an item.

For more information, see "Using Cursor Variables".

Syntax

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Text description of the illustration ref_cursor_type_definition.gif

Keyword and Parameter Description

cursor_name

This identifies an explicit cursor previously declared within the current scope.

cursor_variable_name

This identifies a PL/SQL cursor variable previously declared within the current scope.

db_table_name

This identifies a database table (or view) that must be accessible when the declaration is elaborated.

record_name

This identifies a user-defined record previously declared within the current scope.

record_type_name

This identifies a user-defined record type that was defined using the datatype specifier RECORD.

REF CURSOR

In PL/SQL, pointers have datatype REF X, where REF is short for REFERENCE and X stands for a class of objects. Therefore, cursor variables have datatype REF CURSOR.

RETURN

This keyword introduces the RETURN clause, which specifies the datatype of a cursor variable return value. You can use the %ROWTYPE attribute in the RETURN clause to provide a record type that represents a row in a database table or a row returned by a cursor or strongly typed cursor variable. Also, you can use the %TYPE attribute to provide the datatype of a previously declared record.

%ROWTYPE

This attribute provides a record type that represents a row in a database table or a row fetched from a cursor or strongly typed cursor variable. Fields in the record and corresponding columns in the row have the same names and datatypes.

%TYPE

This attribute provides the datatype of a previously declared user-defined record.

type_name

This is a user-defined cursor variable type that was defined using the datatype specifier REF CURSOR.

Usage Notes

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

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.

REF CURSOR types can be strong (restrictive) or weak (nonrestrictive). A strong REF CURSOR type definition specifies a return type, but a weak definition does not. 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.

Once you define a REF CURSOR type, you can declare cursor variables of that type. You can use %TYPE to provide the datatype of a record variable. Also, 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 strongly (not weakly) typed cursor variable.

Currently, cursor variables are subject to several restrictions. See "Restrictions on 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. When all the rows are processed, you CLOSE the cursor variable.

Other OPEN-FOR statements can open the same cursor variable for different queries. You need not close a cursor variable before reopening it. When you reopen a cursor variable for a different query, the previous query is lost.

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.

If both cursor variables involved in an assignment are strongly typed, they must have the same datatype. However, if one or both cursor variables are weakly typed, they need not have the same datatype.

When declaring a cursor variable as the formal parameter of a subprogram that fetches from or closes the cursor variable, you must specify the IN or IN OUT mode. If the subprogram opens the cursor variable, you must specify the IN OUT mode.

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.

You can apply the cursor attributes %FOUND, %NOTFOUND, %ISOPEN, and %ROWCOUNT to a cursor variable.

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:

OPEN the cursor variable FOR the query.
Assign to the cursor variable the value of an already OPENed host cursor variable or PL/SQL cursor variable.

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.

Examples

You can declare a cursor variable in a PL/SQL host environment such as an OCI or Pro*C program. To use the host 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 a 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.

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 three 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; 
END;

You can also pass a cursor variable to PL/SQL by calling a stored procedure that declares a cursor variable as one of its formal parameters. To centralize data retrieval, you can group type-compatible queries in a packaged procedure, as the following example shows:

CREATE PACKAGE emp_data AS
   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;

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 EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
   TYPE DeptCurTyp IS REF CURSOR RETURN dept%ROWTYPE;
   TYPE BonusCurTyp IS REF CURSOR RETURN bonus%ROWTYPE;
   ...
END cv_types;