11.1. 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