Logical operators

Logical operators

Operator	Description	Example
AND	True if both values are true	a AND b
OR	True if either value is true	a OR b
NOT	True if the value is false	NOT a

Effect of NULL on logical operators

The result of an AND comparison may be NULL if one or both sides of the expression are NULL. If at least one side of an AND operator is FALSE the expression evaluates to FALSE:

    SELECT CAST(null AS boolean) AND true; -- null

    SELECT CAST(null AS boolean) AND false; -- false

    SELECT CAST(null AS boolean) AND CAST(null AS boolean); -- null

The result of an OR comparison may be NULL if one or both sides of the expression are NULL. If at least one side of an OR operator is TRUE the expression evaluates to TRUE:

    SELECT CAST(null AS boolean) OR CAST(null AS boolean); -- null

    SELECT CAST(null AS boolean) OR false; -- null

    SELECT CAST(null AS boolean) OR true; -- true

The following truth table demonstrates the handling of NULL in AND and OR:

a	b	a AND b	a OR b
TRUE	TRUE	TRUE	TRUE
TRUE	FALSE	FALSE	TRUE
TRUE	NULL	NULL	TRUE
FALSE	TRUE	FALSE	TRUE
FALSE	FALSE	FALSE	FALSE
FALSE	NULL	FALSE	NULL
NULL	TRUE	NULL	TRUE
NULL	FALSE	FALSE	NULL
NULL	NULL	NULL	NULL

The logical complement of NULL is NULL as shown in the following example:

    SELECT NOT CAST(null AS boolean); -- null

The following truth table demonstrates the handling of NULL in NOT:

a	NOT a
TRUE	FALSE
FALSE	TRUE
NULL	NULL