Question

I have a database table, which a program is about to update. I have taken a snapshot of the table before the program ran and a snapshot afterwards. Here is a sample of the data:

Before update:

JoinReference   User
1               User 1
1               User 2
1               User 3
2               User 4
2               User 5
2               User 6

After update:

JoinReference   User
3               User 1
3               User 2
3               User 3
4               User 4
4               User 5
5               User 6

I am trying to find all the users that are not linked together after the update. For example, user 1; user 2 and user 3 are linked before and after the update (even though they have a different join reference). User 4 and five are still linked after the update. User 6 is not linked after the update. I am trying to return user 6 after the update. I have tried using derived tables but it has not worked.

This is for a very large database. The example I have given uses fields I have made up to explain the problem. The database structure for the example would be like this:

CREATE TABLE JoinedUsers (JoinReference int, User)

Answer 1

select     UnpairedBefore.userid
from 
           (select    main.*
           from       tableBefore main
           left join  tableBefore ref
           on         main.joinreference = ref.joinreference
           and        main.userid <> ref.userid
           where      ref.userid null) UnpairedBefore
inner join
           (select    main.*
           from       tableAfter main
           left join  tableAfter ref
           on         main.joinreference = ref.joinreference
           and        main.userid <> ref.userid
           where      ref.userid null) UnpairedAfter
on         UnpairedBefore.userid = UnpairedAfter.userid

or

select  [user]
from    tblBefore
where   JoinReference in 
    (
    select  JoinReference
    from    tblBefore
    group by    JoinReference
    having  COUNT(*) = 1
    )
and [user] in 
    (
    select  [user]
    from    tblAfter
    where   JoinReference in 
        (
        select  JoinReference
        from    tblAfter
        group by    JoinReference
        having  COUNT(*) = 1
        )
    )