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