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