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)