Logical Operators

SQL uses a 3-valued logic system with TRUE, FALSE and NULL (unknown):

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

a	NOT a
TRUE	FALSE
FALSE	TRUE
NULL	NULL

The operators AND and OR are commutative. However, it is not guaranteed that the left operand is evaluated before the right.