Why doesn't “WHERE column = NULL” throw an error in SQL Server? [duplicate]

Answer 1

NULL is a special value in RDBMS that means an unknown or inapplicable value.

So nothing will ever be = NULL; such comparison is not valid, thus any queries with expr = NULL in their WHERE clause will return no rows.

Of course this has its quirks and some implementations do things differently; eg: Oracle treats empty strings ('') as NULL and gets quite a bit grief for that.

SQL Server has the ANSI NULL parameter that makes it compliant with SQL-92 (eg comparisons with NULLs like you did won't work.

Answer 2

You can force the behavior by using SET ANSI_NULLS OFF; but since this is deprecated and makes the code rely on a SET setting that can't always be guaranteed, the real solution is to use the right syntax (the one that returns the row).

Answer 3

That is because a NULL can't be equated to any value.

Disable the ANSI_NULLS option and then run it you will see the row now:

SET ANSI_NULLS OFF
select * from #foo --returns the one record we just created  
select * from #foo where colA = null --does not throw an error and does not return a record! why?? 
select * from #foo where colA is null --returns the record  drop table #foo 

Answer 4

I can't tell you why and for what reason the = operator does not support null values but in mysql there is another operator that does: "<=>" Only works in mysql though.

Answer 5

It does not throw an error because it is not an invalid query. The problem is that NULL represents the absence of data and you can't have anything that is equal to that. So when you do

select * from #foo where colA = null

it does try to find rows where colA = null, but that's just impossible. Therefore no errors and no results.