SQL: Get single result id from multiple rows

Question

I have two table which looks like this:

conversations

conversation_id | type    | updated_at
     50         | private | 2018-01-01 15:50:51
     30         | group   | 2019-01-01 15:50:49
    100         | private | 2018-01-01 15:50:55

and

conversation_participants

user_id | conversation_id
1       | 100
3       | 50
5       | 99
6       | 50
6       | 30
3       | 30
2       | 30

How can I now select the conversation_id depending on two user_id and the type of the conversation?

For example, give me the conversation_id from user 3 and 6 which is private. So i would get conversation_id 50.

My approach so far:

SELECT * 
FROM conversation_participants as cp 
    LEFT JOIN conversations as c 
       ON c.conversation_id = cp.conversation_id
 WHERE c.type = 'private' 

But how can I fetch the conversation_id depending on different rows?

Answer 1

First you need to know which converstion are private

SELECT conversation_id 
FROM conversations c
WHERE c.type = 'private' 

Then you need to see what conversation have users 3 and 6 using a SELF JOIN

SELECT conversation_id
FROM conversation_participants cp1
JOIN conversation_participants cp2
  ON cp1.conversation_id = cp2.conversation_id
WHERE cp1 = 3
  AND cp2 = 6 

Now you filter only the private conversations

SELECT conversation_id
FROM conversation_participants cp1
JOIN conversation_participants cp2
  ON cp1.conversation_id = cp2.conversation_id
WHERE conversation_id IN (  SELECT conversation_id 
                            FROM conversations c
                            WHERE c.type = 'private' )
 AND cp1 = 3
 AND cp2 = 6

You can also use a third join

SELECT conversation_id
FROM conversation_participants cp1
JOIN conversation_participants cp2
  ON cp1.conversation_id = cp2.conversation_id
JOIN conversations c
  ON cp1.conversation_id = c.conversation_id   
WHERE c.type = 'private' 
 AND cp1 = 3
 AND cp2 = 6

Answer 2

I would suggest you start by selecting conversations based on your participants. For example:

SELECT * 
FROM conversation_participants cp
WHERE cp.user_id IN (3, 6)

To also include only private conversations, you'll have to join conversation_participants to conversations :

SELECT * 
FROM conversation_participants cp
    INNER JOIN conversations c ON
        c.conversation_id = cp.conversation_id
WHERE
    cp.user_id IN (3, 6)
    AND c.type = 'private'

Of course, this query will include a row for each user in the conversation. If you want to only have 1 row, you can group by conversation. Something like this:

SELECT
    c.conversation_id,
    c.updated_at,
    COUNT(*) AS participant_count
FROM conversation_participants cp
    INNER JOIN conversations c ON
        c.conversation_id = cp.conversation_id
WHERE
    cp.user_id IN (3, 6)
    AND c.type = 'private'
 GROUP BY 
    c.conversation_id,
    c.updated_at

You'll need to use the GROUPING clause, and also add a HAVING clause, to refine your query (especially if you want to search for a conversation specifying more than 2 participants):

SELECT
    c.conversation_id,
    COUNT(*) AS participant_count
FROM conversations c
    INNER JOIN conversation_participants cp ON
        cp.conversation_id = c.conversation_id
WHERE
    c.type = 'private'
    AND cp.user_id IN (3, 6)
 GROUP BY c.conversation_id
 HAVING COUNT(*) = 2