desperate with complicated join query (mysql & php) - php

Question

okay, i'm setting up a multi-user chat system.
i have a messages table, that holds all the messages - simplified:
messages:
id,
content,
thread_id
then i have a threads table:
messages_threads:
id,
participantlist_id
then i have a participantlist table:
participantlist:
list_id,
name
and i have a participantlist_links table:
participantlist_links:
list_id,
participant_id
now i'd like to find out the thread id from a list (array) of participants, i came up with this:
$threadid_sqlquery = "
SELECT messages_threads.id FROM messages_threads
JOIN participantlist_links
ON messages_threads.participantlist_id = participantlist_links.list_id";
foreach($participants_array as &$participant_id){
$maxid_sqlquery .= "
AND participantlist_links.participant_id='".$participant_id."'";
}
but now: i'm looking for the thread_id of a chat between users 1 and 2 - there is a multi-user chat with users 1,2,3 and a chat with users 1,2,3,4 i get a result of three rows, but i only want the one where only users 1 and 2 are participants.
is there any way to query for that?

Answer 1

SELECT mt.id
FROM (
SELECT list_id
FROM participantlist_links pll
WHERE participant_id IN (1, 2)
GROUP BY
list_id
HAVING COUNT(*) = 2
) q
JOIN message_threads mt
ON mt.participantlist_id = q.list_id
WHERE NOT EXISTS
(
SELECT NULL
FROM participantlist_links pll
WHERE pll.list_id = q.list_id
AND pll.participant_id NOT IN (1, 2)
)
Create the following indexes:
participantlist_links (participant_id, list_id) -- unique
participantlist_links (list_id)