The usual logical operators include AND, OR, and NOT. SQL uses a three-valued logical system with true, false, and null, which represents "unknown". Their priorities are NOT > AND > OR.

Table 1 lists operation rules, where a and b represent logical expressions.

**Table 1** Operation rules
a	b	a AND b Result	a OR b Result	NOT a Result
TRUE	TRUE	TRUE	TRUE	FALSE
TRUE	FALSE	FALSE	TRUE	FALSE
TRUE	NULL	NULL	TRUE	FALSE
FALSE	FALSE	FALSE	FALSE	TRUE
FALSE	NULL	FALSE	NULL	TRUE
NULL	NULL	NULL	NULL	NULL

The operators AND and OR are commutative, that is, you can switch the left and right operand without affecting the result.

Logical Operators