Using QMF

Unknown values

QMF interprets the NULL keyword as "unknown". The result of an operation on an unknown value is also unknown, so the result of any operation on NULL is NULL.

Remember that NULL is not zero. NULL is the absence of a value. In the sample table Q.STAFF, there is no value for COMM for managers because managers do not make commissions. In some examples, we calculate earnings as SALARY + COMM. If we did this calculation for managers, the result would always be NULL.

Rules for NULL:

• You can use NULL alone or with =, ¬=, or NOT.
• In a CONDITIONS box, you can use NULL only with a column name or an example element.