MySql query for selecting latest message from the conversation and messages table join

Question

Please tell me what is wrong with this code, this returns all the conversations, but it returns latest message for only first conversation.

    SELECT 
        conversations.*,
        messages.message,
        patients.first_name as fullname,
        patients.city,
        patients.thumb,
        patients.gender,
        patients.online_status
    FROM 
        patients
    INNER JOIN conversations
        ON conversations.patient_id_fk = patients.id
    LEFT JOIN messages
        ON messages.conversation_id = conversations.id
        AND messages.message_id = 
        (
           SELECT MAX(message_id) 
           FROM messages z
           WHERE z.therapist_id_fk = conversations.therapist_id_fk
        )
    WHERE conversations.therapist_id_fk='1'
    GROUP BY conversations.id
    ORDER BY messages.message_id DESC

Answer 1

You want the maximum message for the conversation not the therapist , so I suspect you want this in this subquery:

    AND messages.message_id = 
    (
       SELECT MAX(message_id) 
       FROM messages z
       WHERE z.message_id = conversations.id
    )

I'm not sure the outer GROUP BY is correct (it may not be needed at all and it seems to conflict with the columns of the SELECT ), but without sample data and desired results, it is hard to tell.

EDIT:

I see, the problem is the filtering on therapist id in the outer query. That makes this a bit more complicated:

    AND messages.message_id = 
    (
       SELECT MAX(message_id) 
       FROM messages z JOIN
            conversations c
            ON m.conversation_id = c.id
       WHERE c.therapist_id_fk = 1 AND  -- restriction on subquery
             z.message_id = conversations.id  -- correlation to outer query
    )

Don't use single quotes for constants unless the column is a string or date.

Answer 2

    SELECT 
        conversations.*,
        messages.message,
        patients.first_name as fullname,
        patients.city,
        patients.thumb,
        patients.gender,
        patients.online_status
    FROM 
        patients
    INNER JOIN conversations
        ON conversations.patient_id_fk = patients.id
    LEFT JOIN messages
        ON messages.conversation_id = conversations.id
        AND messages.message_id = 
        (
           SELECT MAX(message_id) 
           FROM messages z
           WHERE z.conversation_id = conversations.id
        )
    WHERE conversations.therapist_id_fk='1'
    ORDER BY messages.message_id DESC

Found the issue. It was here:

AND messages.message_id = 
            (
               SELECT MAX(message_id) 
               FROM messages z
               WHERE z.conversation_id = conversations.id
            )

I was using this:

AND messages.message_id = 
        (
           SELECT MAX(message_id) 
           FROM messages z
           WHERE z.therapist_id_fk = conversations.therapist_id_fk
        )

Answer 3

I was able to solve the problem with this query

SELECT conversations.*,m.message FROM `conversations` LEFT JOIN messages as m on conversations.id = m.conversation_id AND m.id = (SELECT MAX(ms.id) from messages as ms where ms.conversation_id = conversations.id)

I left join the messages and when doing so i select the top most message. Order by can be used to give the order by date

Answer 4

Your problem is the bit:

SELECT MAX(message_id) 
FROM messages z
WHERE z.therapist_id_fk = conversations.therapist_id_fk

This query only matches exactly one message. You'd need something a bit different. Try this:

SELECT 
    conversations.*,
    messages.message,
    patients.first_name as fullname,
    patients.city,
    patients.thumb,
    patients.gender,
    patients.online_status
FROM 
    patients
INNER JOIN conversations
    ON conversations.patient_id_fk = patients.id
LEFT JOIN messages
    ON messages.conversation_id = conversations.id        
WHERE conversations.therapist_id_fk='1' 
      AND messages.message_id IN (  -- new bit
          SELECT MAX(message_id) FROM messages z GROUP BY z.conversation_id
      )
GROUP BY conversations.id
ORDER BY messages.message_id DESC