# 8.1. 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