FALSE is a synonym of 0 and
TRUE is a synonym of 1. These constants are case insensitive, so
true are equivalent.
These terms are not synonyms of 0 and 1 when used with the
IS operator. So, for example,
10 IS TRUE returns 1, while
10 = TRUE returns 0 (because 1 != 10).
IS operator accepts a third constant exists:
UNKNOWN. It is always a synonym of
FALSE are reserved words, while
UNKNOWN is not.