View source for Sql Server 2012 - Three-Valued Logic

The type of logic used in such a
case is known as two-valued logic. The WHERE filter returns only the rows for which the predicate
evaluates to true<br>
A predicate can evaluate
to true, false, and unknown. This type of logic is known as three-valued logic<br>
When region is NULL, the predicate region <>   <br>
'WA' evaluates to unknown, and the row is discarded