Null Values Undefined values, or "undef", can be used to indicate null values. However, care must be taken in the particular case of trying to use null values to qualify a "SELECT" statement. Consider: SELECT description FROM products WHERE product_code = ? Binding an "undef" (NULL) to the placeholder will not select rows which have a NULL "product_code"! Refer to the SQL manual for your database engine or any SQL book for the reasons for this. To explicitly select NULLs you have to say ""WHERE product_code IS NULL"" and to make that general you have to say: ... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL)) and bind the same value to both placeholders. Sadly, that more general syntax doesn't work for Sybase and MS SQL Server. However on those two servers the original ""product_code = ?"" syntax works for binding nulls.