Determining the Characteristics of a Result Set

Meta data is data that describes other data. For example, result set meta data describes the characteristics of a result set, such as the number of columns in the result set, the data types of those columns, their names, precision, nullability, and so on.

ODBC supplies meta data to applications through its catalog API functions. The Microsoft® SQL Server™ ODBC driver implements many of the ODBC API catalog functions as calls to a corresponding SQL Server catalog procedure.

Applications require meta data for most result set operations. For example, the application uses the data type of a column to determine what kind of variable to bind to that column. It uses the byte length of a character column to determine how much space it needs to display data from that column. How an application determines the meta data for a column depends on the type of the application.

Vertical applications typically work with predefined tables and perform predefined operations on those tables. Because the result set meta data for such applications is defined before the application is even written and is controlled by the application developer, it can be hard-coded into the application. For example, if an order ID column is defined as a 4-byte integer in the data source, the application can always bind a 4-byte integer to that column. When meta data is hard-coded in the application, a change to the tables used by the application generally implies a change to the application code.

Generic applications, especially applications that support ad hoc queries, almost never know the meta data of the result sets they create. Therefore, they must discover the meta data at run time.

To determine the characteristics of a result set, an application can call:

• SQLNumResultCols to determine how many columns a request returned.
  
  
• SQLColAttribute or SQLDescribeCol to describe a column in the result set.

A well-designed application is written with the assumption that the result set is unknown and uses the information returned by these functions to bind the columns in the result set. An application can call these functions at any time after a statement is prepared or executed. However, for optimal performance, an application should call SQLColAttribute, SQLDescribeCol, and SQLNumResultCols after a statement is executed.

You can have multiple concurrent calls for meta data. The system catalog procedures underlying the ODBC catalog API implementations can be called by the ODBC driver while it is using static server cursors. This allows applications to concurrently process multiple calls to ODBC catalog functions.

If an application uses a particular set of meta data more than once, it will probably benefit by caching the information in private variables when it is first obtained. This eliminates the overhead of later calls to the ODBC catalog functions for the same information (which forces the driver to make roundtrips to the server).

To retrieve result set information