MDAC 2.5 SDK - ODBC Programmer's Reference
Appendix F: ODBC Cursor Library
When an application calls SQLRowCount with the statement associated with the cursor, the cursor library returns the number of rows of data it has retrieved from the driver.
When an application calls SQLRowCount with the statement associated with a positioned update or delete statement, the cursor library returns the number of rows affected by the statement.
When an application calls SQLRowCount after a SELECT statement, the cursor library returns –1.