Why does NULL = NULL evaluate to false in SQL server

Think of the null as “unknown” in that case (or “does not exist”). In either of those cases, you can’t say that they are equal, because you don’t know the value of either of them. So, null=null evaluates to not true (false or null, depending on your system), because you don’t know the values to say that they ARE equal. This behavior is defined in the ANSI SQL-92 standard.

EDIT: This depends on your ansi_nulls setting. if you have ANSI_NULLS off, this WILL evaluate to true. Run the following code for an example…

set ansi_nulls off

if null = null
    print 'true'
else
    print 'false'


set ansi_nulls ON

if null = null
    print 'true'
else
    print 'false'