Re: 3 value logic. Why is SQL so special?

>>, what would be the consequences of NULL=NULL being true?

 NULL=NULL should not be true.
 NULL=NULL should not be false.
 NULL=NULL should not be UNKNOWN.
 NULL=NULL should be NULL. <<

NULL is a missing *attribute* value; UNKNOWN is a *logical* value. The first rule of NULLs is that they propagate. You can easily set up contradictions that depend on the order evaluation when you have a BOOLEAN data type. All SQLK data types must allow NULLs by definition.

NULL OR TRUE = NULL -- by definition
UNKNOWN OR TRUE = TRUE -- by definition

NULL AND TRUE = NULL -- by definition
UNKNOWN AND TRUE = UNKNOWN -- by definition

This is why we have the <exp> IS [NOT] [TRUE | FALSE |UNKNOWN] predicate in SQL-92 Received on Mon Sep 18 2006 - 21:14:59 CDT