含义：mssql_rows_affected返回-1？

This might be a very silly question. I am trying to execute a query using PHP and MSSQL.

BEGIN 
IF NOT EXISTS(SELECT * FROM Table1 WHERE Col1 = 170 AND Col2 = 'Test') 
INSERT INTO Table1 (Col1, Col2) 
VALUES(170, 'Test') 
END

when called mssql_rows_affected() after executing the query it returns -1.

When I execute same query on database directly I get message Command(s) completed successfully. That means my query have no errors (though it is not affecting any records).

Additionally return code for the query is 1 (I hope that means query executed successfully)

I have looked on PHP manual but it says Returns the number of records affected by last operation. It doesn't mention what if it returns -1.

Does anybody know what mssql_rows_affected() = -1 means?

Thanks in advance.

Regards, Ganesh.