Mysql - find conversation only being held by two users

Question

I have a table called participants with the following fields:

• id
• user_id
• conversation_id

The background is that there ar

Answer 1

select conversation_id
from participants
where user_id in (1,2)
group by conversation_id
having count(user_id) = 2

Return two users conversation_id

Answer 2

Using your query will not work since the where clause filters out the user_ids. Use

SELECT * FROM participants
GROUP BY conversation_id
HAVING sum(user_id not in (1,2)) = 0

user_id not in (1,2) returns 1 if a user_id other than 1,2 are in a conversation and 0 otherwise. So using SUM you can add up all that cases. If none are found then the sum is 0.