Question

There’s a strange behaviour that is drive me crazy.

I’ve a table of users and a table of permissions in an old SQL Server 2000 database.
This database is a mess, many tables have no PK and there are no relations between the tables… but I couldn’t fix it (and honestly I don’t think this is related to the problem I have).

Users:
IDRecord -> PK money
-- other fields

Permissions:
IDRecord -> money (is not a PK)
IDUser   -> money (refers to Users.IDRecord WITHOUT FK)
Function -> varchar
-- other fields

I want to get the User’s ids of the users without any permission.

My first approach was to write something as:

select distinct IDRecord 
from Users
where IDRecord not in (
    select IDUser from Permissions
)

That returns me no rows.

But I KNOW there are users without permissions, so I write a second query:

select distinct U.IDRecord 
from Users U
left join Permissions P
    on P.IDUser = U.IDRecord
where P.IDRecord is null

that correctly returns the users without permissions.

So, where’s the problem?

Why the first doesn’t work?

Answer 1

This is expected behavior.
It is expected because SQL has three-valued logic.
In other words: for those users who have no permissions, there is no result (NULL) returned by your subquery.
Your WHERE condition is not satisfied in those cases because a value can never equal nor not equal NULL.

Alternatives:
1) use a LEFT JOIN (as you have done), or
2) use NOT EXISTS, e.g.:

SELECT DISTINCT IDRecord 
  FROM Users u
 WHERE NOT EXISTS (
         SELECT 1 
           FROM Permissions p 
          WHERE p.IDUser = u.IDRecord
       );

---

Edit: More detail on how 3VL can bite you if you’re not careful:
A possibly counter-intuitive result occurs if you do something like this…

...
WHERE a_column <> 'some value';

Suddenly rows where a_column is NULL disappear from your results.
To get them back you can do this:

...
WHERE (a_column <> 'some value' OR a_column IS NULL);