Joe Celko's SQL for Smarties, 3rd Edition

10.2. IS [NOT]{TRUE | FALSE | UNKNOWN} Predicate

This predicate tests a condition that has the truth-value TRUE, FALSE, or UNKNOWN, and returns TRUE or FALSE. The syntax is:

<Boolean test> ::=
    <Boolean primary> [IS [NOT] <truth value>]

<truth value> ::= TRUE | FALSE | UNKNOWN

<Boolean primary> ::=
    <predicate> | <left paren> <search condition> <right paren>

As you would expect, the expression IS NOT <logical value> is the same as NOT (IS <logical value>), so the predicate can be defined as shown in Table 10.2.

Table 10.2. Defining the Predicate: True, False, or Unknown

IS condition | TRUE FALSE UNKNOWN ======================================= TRUE | TRUE FALSE FALSE FALSE | FALSE TRUE FALSE UNKNOWN | FALSE FALSE TRUE

If you are familiar with ...