Question

I have three tables, user, conversation, and conversation_participant with the rows:

user
id

conversation
id
type

conversation_participant
conversation_id
user_id

(type indicates that it is a “private,” 1-1 conversation, i.e., ensuring that user1 and user2’s conversation is delineated from a group conversation involving, for example, user1, user2, and user3. type=0 is private, type=1 is a group.)

Regardless, how can I best structure a query that determines if a private conversation between two users (user_id=1 and user_id=2) exists? I am tempted to do something like this (I am very new to SQL, mind you):
SELECT conversation_participant.conversation_id FROM conversation_participant WHERE user_id=1 …but I’m pretty certain this is off to an unfortunate start.

What is the most concise way to manage a situation like this with queries from multiple tables that are all interconnected in this fashion? I assume it’s relatively simple, and if there are any good readings on this that you may know of, they would be much appreciated as well.

Thank you!

Answer 1

If I understand correctly that you are looking for conversations between two specific known user ids:

select c.id
from conversation_participant cp1
inner join conversation_participant cp2
    on cp1.conversation_id=cp2.conversation_id and cp2.user_id=2
inner join conversation c
    on cp1.conversation_id=c.id and c.type=0
where cp1.user_id=1;

In English, find ids of conversations where:

user_id 1 is a participant (from and where clauses)

and user_id 2 is a participant in the same conversation (first join clause)

and that conversation is of private type (second join clause).