DECLARE (result set cursor)

Associates a database cursor with an SQL statement producing a result set.

Syntax 1: Cursor declared with a static SQL statement.

DECLARE cid [SCROLL] CURSOR [WITH HOLD] FOR select-statement
  1. cid is the identifier of the database cursor.
  2. select-statement is a SELECT statement defined in static SQL.

Syntax 2: Cursor declared with a prepared statement.

DECLARE cid [SCROLL] CURSOR [WITH HOLD] FOR sid 
  1. cid is the identifier of the database cursor.
  2. sid is the identifier of a prepared SQL statement.

Syntax 3: Cursor declared with a string expression.

DECLARE cid [SCROLL] CURSOR [WITH HOLD] FROM expr
  1. cid is the identifier of the database cursor.
  2. expr is any expression that evaluates to a string.

Syntax 4: Cursor declared with an SQL Block.

DECLARE cid [SCROLL] CURSOR [WITH HOLD] FOR SQL sql-statement END SQL
  1. cid is the identifier of the database cursor.
  2. sql-statement is a statement defined in an SQL block.

Usage

The DECLARE instruction allocates resources for an SQL statement handle, in the context of the current connection. The SQL text is sent to the database server for parsing, validation and to generate the execution plan.

After declaring the cursor, you can use the OPEN instruction to execute the SQL statement and produce the result set. Rows can be fetched with the FETCH instruction or in a FOREACH loop.

DECLARE must precede any other statement that refers to the cursor during program execution.

The scope of reference of the cid cursor identifier is local to the module where it is declared.

Resources allocated by the DECLARE cursor-name can be released later by the FREE cursor-name instruction.

The static select-statement used in the DECLARE can contain program variables, or ? (question mark) parameter placeholders, that can be bound to program variables with the USING clause of the OPEN instruction.

Important:

When using program variables in a static SQL statement of a DECLARE CURSOR instruction, make sure that these variables are available when executing the OPEN or FOREACH instruction. Otherwise, the behavior is unexpected and can make the runtime system crash. Do not mix ? SQL parameter placeholders with program variables: This would lead to a runtime error at OPEN or at FOREACH time, because the number of SQL parameters will not match the number of variable provided in the USING clause. The fglcomp -S option extracts the static SQL statements showing ? placeholders instead of programs variables. This can help to check the actual number of SQL parameters in a static SQL statement.

The maximum number of declared cursors in a single program is limited by the database server and the available memory. Make sure that you free the resources when you no longer need the declared cursor.

When declaring a cursor with a static select-statement, the statement can include an INTO clause. However, to be consistent with prepared statements you better omit the INTO clause in the SQL text and use the INTO clause of the FETCH statement to retrieve the values from the result set.

Add the FOR UPDATE clause to the SELECT statement to declare an update cursor. You can then use the update cursor to modify (update or delete) the current row with WHERE CURRENT OF.

Use the WITH HOLD option with care, because this feature is specific to IBM® Informix® servers. Other database servers do not behave as Informix does with this type of cursor. For example, if the SELECT is not declared FOR UPDATE, most database servers keep cursors open after the end of a transaction, but IBM DB2® automatically closes all cursors when the transaction is rolled back.

Forward only cursors

If you use only the DECLARE CURSOR keywords, you create a sequential cursor, which can fetch only the next row in sequence from the result set. The sequential cursor can read through the result set only once each time it is opened. If you are using a sequential cursor for a select cursor, on each execution of the FETCH statement, the database server returns the contents of the current row and locates the next row in the result set.

Cursors can be declare with a static SELECT statement:
MAIN
   DATABASE stores
   DECLARE c1 CURSOR FOR SELECT * FROM customer 
END MAIN
Cursors can also be declared with a SELECT statement defined in a character string:
MAIN
  DEFINE key INTEGER
  DEFINE cust RECORD
           num INTEGER,
           name CHAR(50)
        END RECORD
  DATABASE stores
  DECLARE c1 CURSOR FOR
      SELECT customer_num, cust_name FROM customer WHERE customer_num > ?
  LET key=101
  FOREACH c1 USING key INTO cust.*
     DISPLAY cust.*
  END FOREACH
END MAIN

Scrollable cursors

Use the DECLARE SCROLL CURSOR keywords to create a scrollable cursor, which can fetch rows of the result set in any sequence. Until the cursor is closed, the database server retains the result set of the cursor in a static data set (for example, in a temporary table like Informix). You can fetch the first, last, or any intermediate rows of the result set as well as fetch rows repeatedly without having to close and reopen the cursor. On a multiuser system, the rows in the tables from which the result set rows were derived might change after the cursor is opened and a copy of the row is made in the static data set. If you use a scroll cursor within a transaction, you can prevent copied rows from changing, either by setting the isolation level to REPEATABLE READ or by locking the entire table in share mode during the transaction. Scrollable cursors cannot be declared FOR UPDATE.

With most database servers, scrollable cursors take quite a few resources to hold a static copy of the result set. Therefore you should consider optimizing scrollable cursor usage by fetching only the primary keys of rows, and execute a secondary SELECT statement to fetch other fields for each row that must be displayed.

The DECLARE [SCROLL] CURSOR FROM syntax allows you to declare a cursor directly with a string expression, so that you do not have to use the PREPARE instruction. This simplifies the source code and speeds up the execution time for non-Informix databases, because the SQL statement is not parsed twice.

MAIN
  DEFINE key INTEGER
  DEFINE cust RECORD
           num INTEGER,
           name CHAR(50)
       END RECORD
  DATABASE stores
  DECLARE c1 SCROLL CURSOR FOR
      SELECT customer_num, cust_name FROM customer WHERE customer_num > ?
  LET key=101
  FOREACH c1 USING key INTO cust.*
     DISPLAY cust.*
  END FOREACH
END MAIN

Hold cursors

Use the WITH HOLD option to create a hold cursor. A hold cursor allows uninterrupted access to a set of rows across multiple transactions. Ordinarily, all cursors close at the end of a transaction. A hold cursor does not close; it remains open after a transaction ends. A hold cursor can be either a sequential cursor or a scrollable cursor.

Note:

WITH HOLD cursors are fully supported by Informix database engines, but these must be used with care to write portable SQL code: Most database engines keep cursors open across transactions (with hold cursor is the default), and the ODI drivers emulate the Informix behavior by closing all cursors not declared WITH HOLD at the end of a transaction. But each database brand can have a specific behavior and implicitly close the SQL cursor, when rolling the transaction back, or when combining hold cursor option with SELECT ... FOR UPDATE. See the SQL programming topic about WITH HOLD cursors.

MAIN
  DEFINE key INTEGER
  DEFINE cust RECORD
           num INTEGER,
           name CHAR(50)
       END RECORD
  DATABASE stores 
  DECLARE c1 CURSOR WITH HOLD FOR
      SELECT customer_num, cust_name FROM customer WHERE customer_num > ?
  LET key=101
  FOREACH c1 USING key INTO cust.*
     BEGIN WORK
     UPDATE cust2 SET name=cust.cust_name WHERE num=cust.num 
     COMMIT WORK
  END FOREACH
END MAIN