Three-valued logic

The following tables display how the AND, OR, NOT, and IS logical operators of SQL work in three-valued logic.

AND operator
AND TRUE FALSE UNKNOWN
TRUE TRUE FALSE UNKNOWN
FALSE FALSE FALSE FALSE
UNKNOWN UNKNOWN FALSE UNKNOWN
OR operator
OR TRUE FALSE UNKNOWN
TRUE TRUE TRUE TRUE
FALSE TRUE FALSE UNKNOWN
UNKNOWN TRUE UNKNOWN UNKNOWN
NOT operator
TRUE FALSE UNKNOWN
FALSE TRUE UNKNOWN
IS operator
IS TRUE FALSE UNKNOWN
TRUE TRUE FALSE FALSE
FALSE FALSE TRUE FALSE
UNKNOWN FALSE FALSE TRUE
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