SQL Language Guide : 4. Elements of SQL Statements : Predicates in SQL : IS TRUE, IS FALSE, IS UNKNOWN Predicates
 
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IS TRUE, IS FALSE, IS UNKNOWN Predicates
The IS Boolean operator behaves according to the following truth table:
IS
TRUE
FALSE
UNKNOWN
True
True
False
False
False
False
True
False
Unknown
False
False
True
That is:
IS TRUE is true for a BOOLEAN TRUE.
IS FALSE is true for a BOOLEAN FALSE.
IS UNKNOWN is true for an unknown (NULL) value.
Every other comparison is false.