Question

(not discussing the table design, its a done deal!)

I know how to do this query using a WHERE NOT IN clause, but I was wondering how it could be done using a join.

Note: rows are not unique, so we have to do a DISTINCT so as to not get duplicates.

Users (userID)
OldUsers(UserID)

So using the WHERE NOT IN clause I can do:

SELECT DISTINCT(u.userID)
FROM Users u
WHERE u.userID NOT IN (SELECT o.userID FROM OldUsers o)

Performance is a key here also.

Answer 1

select distinct u.userID from Users u
left join OldUsers o on o.userId = u.userID
where o.userId is null