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NULL Value Comparisons

To compare for the null value, use IS NULL. To compare for any value but the null value, use IS NOT NULL. For example:

salary IS NOT NULL

The following rules govern null values in an expression:

• A NULL value in an expression with relational or arithmetic operators causes the expression to evaluate to NULL.

• If you use the logical AND operator and either expression is FALSE, then the result is FALSE. If one expression is TRUE and the other is NULL, then the result is NULL. The result is only TRUE if both expressions are TRUE.

• If you use the logical OR operator and either expression is TRUE, then the result is TRUE. If one expression is FALSE and the other is NULL, the expression is NULL. The result is FALSE only if both expressions are FALSE.

If you use the logical NOT operator with an expression that evaluates to a null value, the result is still null.