SQL - Linked information - sql

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
)
)

Answer 2

You want something along the lines of
SELECT *
FROM myTable
WHERE (row_was_updated)
AND JoinReference IN
(SELECT JoinReference
FROM myTable
GROUP BY JoinReference
HAVING COUNT(*) = 1)
You may or may not need WHERE (row_was_updated) and you'll have to fill in that logic yourself if you need it.
It's likely there would be a more efficient way of doing that, but without schema it's only possible to give general guidance.

Answer 3

SELECT b1.user AS user1
, b2.user AS user2
FROM BeforeTable AS b1 --- user1 and
JOIN BeforeTable AS b2 --- user2 were linked
ON b1.JoinReference = b2.JoinReference --- before the update
WHERE b1.user < b2.user
AND NOT EXISTS --- and are not linked
( SELECT *
FROM AfterTable AS a1 --- after the update
JOIN AfterTable AS a2
ON a1.JoinReference = a2.JoinReference
WHERE a1.user = b1.user
AND a2.user = b2.user
) ;