Avoid problems with three-valued logic.
By Lex de Haan and Jonathan Gennick
July/August 2005
Called null values in the ISO SQL standard, nulls are anything
but values. Nulls are markers indicating the complete lack of a
value. They lead to three-valued logic, which is confusing to work
with, and that confusion often leads the unwary down the path of
writing SELECT statements that return wrong results. This
article highlights some of the pitfalls you'll encounter and gives
advice...
