Question

I have a MessageThread table

UserUid   ThreadUid
===================
K1    111
N1    111
K1    222
K1    333
N1    444
R1    111
R1    222
D1    333
E1    444
T1    555

One threadUid is only used for a same group of users.

K1 and N1 can on only have same ThreadUid.

if I know UserUid: K1 and N1

How can I know if K1 and N1 has a same ThreadUid?

Is there a way to get ThreadUid based on the provided UserUid? Seems impossible…

————

If the UserUid is K1 and N1
Result will be 111

If the UserUid is K1 and T1
Result will be empty

If the UserUid is N1 and E1
Result will be 444

Answer 1

To find threads that both users have in common:

select  mt1.ThreadUid
,       count(*) as ThreadCount
,       (
        select  count(distinct mt3.UserUid) 
        from    MessageThread mt3 
        where   mt1.ThreadUid = mt1.ThreadUid
        ) as UsersInThread
from    MessageThread mt1
join    MessageThread mt2
on      mt1.ThreadUid = mt2.ThreadUid
where   mt1.UserUid = 'K1'
        and mt2.UserUid = 'N1'
group by
        mt1.ThreadUid

Example at SQL Fiddle.