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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](#Operation rules) lists the calculation 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

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