瞧一瞧迷一般的SQLDA

With static SQL, host variables used in embedded SQL statements are known at application compile time. With dynamic SQL, the embedded SQL statements and consequently the host variables are not known until application run time. Therefore, for dynamic SQL applications, you must preprocess the list of host variables that are used in your application.

You can use the DESCRIBE statement to obtain host variable information for any SELECT statement that has been prepared (using PREPARE), and store that information into the SQL descriptor area (SQLDA).

When the DESCRIBE statement gets executed in your application, the database manager defines your host variables in an SQLDA. Once the host variables are defined in the SQLDA, you can use the FETCH statement to assign values to the host variables, using a cursor.

• Declaring the SQLDA structure in a dynamically executed SQL program
  An SQLDA contains a variable number of occurrences of SQLVAR entries, each of which contains a set of fields that describe one column in a row of data. There are two types of SQLVAR entries: base SQLVAR entries and secondary SQLVAR entries.
• Preparing a dynamically executed SQL statement using the minimum SQLDA structure
  Use the information provided here as an example of how to allocate the minimum SQLDA structure for a statement.
• Allocating an SQLDA structure with sufficient SQLVAR entries for dynamically executed SQL statements
  After you determine the number of columns in the result table, you must allocate storage for a second full-size SQLDA. The first SQLDA is used for input parameters and the second full-size SQLDA is used for output parameters.
• Describing a SELECT statement in a dynamically executed SQL program
  After you allocate sufficient space for the second SQLDA (in this example, calledfulsqlda), you must code the application to describe the SELECT statement.
• Acquiring storage to hold a row
  Before the application can fetch a row of the result table using an SQLDA structure, the application must first allocate storage for the row.
• Processing the cursor in a dynamically executed SQL program
  After you allocate the SQLDA structure, you can open the cursor associated with the SELECT statement and fetch rows.
• Allocating an SQLDA structure for a dynamically executed SQL program
  Allocate an SQLDA structure for your application so that you can use it to pass data to and from your application.
• Transferring data in a dynamically executed SQL program using an SQLDA structure
  You have greater flexibility when you transfer data using an SQLDA instead of using lists of host variables. For example, you can use an SQLDA to transfer data that has no native host language equivalent, such as DECIMAL data in the C language.
• Processing interactive SQL statements in dynamically executed SQL programs
  You can write an application using dynamic SQL to process arbitrary SQL statements. For example, if an application accepts SQL statements from a user, the application must be able to issue the statements without any prior knowledge of the statements.
• Determination of statement type in dynamically executed SQL programs
  When an SQL statement is prepared, you can determine information concerning the type of statement by examining the SQLDA structure. This information is placed in the SQLDA structure either at statement preparation time with the INTO clause, or by issuing a DESCRIBE statement against a previously prepared statement.
• Processing variable-list SELECT statements in dynamically executed SQL programs
  A varying-list SELECT statement is one in which the number and types of columns that are to be returned are not known at precompilation time.
• Saving SQL requests from end users
  If the users of your application can issue SQL requests from the application, you might want to save these requests.

 

Related reference:

DESCRIBE INPUT statement

FETCH statement

PREPARE statement

SQLDA data structure