Process SELECT statements and use a descriptor

There are two basic types of SELECT statements: fixed list and varying list.

To process a fixed-list SELECT statement, an SQL descriptor is not necessary.

Start of change To process a varying-list SELECT statement, you must first declare an SQL descriptor area (SQLDA) structure or ALLOCATE an SQLDA. Both forms of SQL descriptors can be used to pass host variable input values from an application program to SQL and to receive output values from SQL. In addition, information about SELECT list expressions can be returned in a PREPARE or DESCRIBE statement. End of change

Fixed-list SELECT statements
In dynamic SQL, fixed-list SELECT statements are those statements designed to retrieve data of a predictable number and type. When using these statements, you can anticipate and define host variables to accommodate the retrieved data so that an SQL descriptor area (SQLDA) is not necessary.
Varying-list SELECT statements
In dynamic SQL, varying-list SELECT statements are ones for which the number and format of result columns to be returned are not predictable; that is, you do not know how many variables you need, or what the data types are.
SQL descriptor areas
Dynamic SQL uses an SQL descriptor area (SQLDA) to pass information about an SQL statement between SQL and your application. A descriptor is required for running the DESCRIBE, DESCRIBE INPUT and DESCRIBE TABLE statements, and can also be used on the PREPARE, OPEN, FETCH, CALL, and EXECUTE statements.
SQLDA format
The SQLDA consists of four variables followed by an arbitrary number of occurrences of a sequence of six variables collectively named SQLVAR.
Example: Select-statement for allocating storage for SQLDA
Suppose your application needs to be able to handle a dynamic SELECT statement, one that changes from one use to the next. This statement can be read from a display, passed in from another application, or built dynamically by your application.
Example: Select statement using an allocated SQL descriptor
Suppose your application needs to be able to handle a dynamic SELECT statement; one that changes from one use to the next. This statement can be read from a display, passed from another application, or built dynamically by your application.
Parameter markers
In the example used, the SELECT statement that was dynamically run had a constant value in the WHERE clause.

Parent topic: Dynamic SQL applications