Question

this is my conversation table:

conversationID  ||  userID
      1         ||    1
      1         ||    2
      2         ||    1
      2         ||    2
      2         ||    3

as you can see each conversation can contain 2 users or more.

I am trying to get the id of the conversation that only 2 users are there.
ie conversation that contains only users 1 & 2, the answer is conversation 1.

but how do I get it?

Answer 1

This will select all conversations that have users 1 or user 2, or both, but no one else:

select conversationID
from conversations
group by conversationID
having count(*) = count(case when userID in (1,2) then 1 end)

If you also want all conversations that have exactly user 1 and 2, and no one else, you also have to add an and condition:

select conversationID
from conversations
group by conversationID
having count(*) = count(case when userID in (1,2) then 1 end)
       and count(*) = 2 -- number of elements in set

If userID can be duplicated, it’s also better to use distinct:

select conversationID
from conversations
group by conversationID
having
  count(distinct userID) = count(distinct case when userID in (1,2) then userID end)
  and count(distinct userID) = 2 -- number of elements in set