Why does NULL = NULL evaluate to false in SQL server

Question

In SQL server if you have nullParam=NULL in a where clause, it always evaluates to false. This is counterintuitive and has caused me many errors. I do understa

Answer 1

MSDN has a nice descriptive article on nulls and the three state logic that they engender.

In short, the SQL92 spec defines NULL as unknown, and NULL used in the following operators causes unexpected results for the uninitiated:

= operator NULL   true   false 
NULL       NULL   NULL   NULL
true       NULL   true   false
false      NULL   false  true

and op     NULL   true   false 
NULL       NULL   NULL   false
true       NULL   true   false
false      false  false  false

or op      NULL   true   false 
NULL       NULL   true   NULL
true       true   true   true
false      NULL   true   false