Appendix D. SQLDA (SQL descriptor area)
An SQLDA is a set of variables that is used for execution of the SQL DESCRIBE statement, and it may optionally be used by the PREPARE, OPEN, CALL, FETCH, and EXECUTE statements. An SQLDA can be used in a DESCRIBE or PREPARE statement, altered with the addresses of storage areas106, and then used again in a FETCH statement.
SQLDAs are supported for all languages, but predefined declarations are provided only for C (and C++), COBOL, ILE RPG, PL/I, and REXX. In REXX, the SQLDA is somewhat different than in the other languages; for information on the use of SQLDAs in REXX, see the Embedded SQL Programming book.
The meaning of the information in an SQLDA depends on its use.
- When an SQLDA is used in a DESCRIBE or PREPARE statement, an SQLDA provides information to an application program about a prepared select-statement. Each column of the result table is described in an SQLVAR occurrence or set of related SQLVAR occurrences.
- In OPEN, EXECUTE, CALL, and FETCH, an SQLDA provides information to the database manager about storage areas for input or output data. Each storage area is described in the SQLVARs.
- For OPEN and EXECUTE of a statement other than CALL, each SQLVAR occurrence or set of related SQLVAR occurrences describes a storage area that is used to contain an input value which is substituted for a parameter marker in the associated SQL statement that was previously prepared.
- For FETCH, each SQLVAR occurrence or set of related SQLVAR occurrences describes a storage area that is used to contain an output value from a row of the result table.
- For CALL and EXECUTE of a prepared CALL statement, each SQLVAR occurrence or set of related SQLVAR occurrences describes a storage area that is used to contain an input or output value (or both) that corresponds to an argument in the argument list for the procedure.
An SQLDA consists of four variables in a header followed by an arbitrary number of occurrences of a base SQLVAR. When the SQLDA desribes either LOBs or distint types the base SQLVARs are followed by the same number of occurrences of an extended SQLVAR.
- Base SQLVAR entry
- The base SQLVAR entry is always present. The fields of this entry contain the base information about the column or variable such as data type code, length attribute (except for LOBs), column name (or label), CCSID, variable address, and indicator variable address.
- Extended SQLVAR entry
- The extended SQLVAR entry is needed (for each column) if the result includes any LOB or distinct type columns. For distinct types, the extended SQLVAR contains the distinct type name. For LOBs, the extended SQLVAR contains the length attribute of the variable and a pointer to the buffer that contains the actual length. If locators or file reference variables are used to represent LOBs, an extended SQLVAR is not necessary.
The extended SQLVAR entry is also needed for each column when:
- USING BOTH is specified, which indicates that column names and labels are returned.
- USING ALL is specified, which indicates that column names, labels, and system column names are returned.
The fields in the extended SQLVAR that return LOB and distinct type information do not overlap, and the fields that return LOB and label information do not overlap. Depending on the combination of labels, LOBs and distinct types, more than one extended SQLVAR entry per column may be required to return the information. See Determining how many SQLVAR occurrences are needed.
- Field descriptions in an SQLDA header
- Field descriptions in an occurrence of SQLVAR
- SQLTYPE and SQLLEN
- CCSID values in SQLDATA or SQLNAME
- Unrecognized and unsupported SQLTYPES
- INCLUDE SQLDA declarations
106. A storage area could be the storage for a variable defined in the program (that may also be a host variable) or an area of storage explicitly allocated by the application.
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