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

Answer 2

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'

Answer 3

If you are looking for an expression returning true for two NULLs you can use:

SELECT 1 
WHERE EXISTS (
    SELECT NULL
    INTERSECT
    SELECT NULL
)

It is helpful if you want to replicate data from one table to another.

Answer 4

Just an addition to other wonderful answers:

AND: The result of true and unknown is unknown, false and unknown is false,
while unknown and unknown is unknown.

OR: The result of true or unknown is true, false or unknown is unknown, while unknown or unknown is unknown.

NOT: The result of not unknown is unknown

Answer 5

To quote the Christmas analogy again:

In SQL, NULL basically means "closed box" (unknown). So, the result of comparing two closed boxes will also be unknown (null).

I understand, for a developer, this is counter-intuitive, because in programming languages, often NULL rather means "empty box" (known). And comparing two empty boxes will naturally yield true / equal.

This is why JavaScript for example distinguishes between null and undefined.

Answer 6

NULL isn't equal to anything, not even itself. My personal solution to understanding the behavior of NULL is to avoid using it as much as possible :).