sqlite3 query by max and filter by second factor

Question

I have:

TABLE MESSAGES
 message_id | conversation_id | from_user | timestamp  |  message

I want:

1. SELECT * WHERE from_user &l         


        

Answer 1

Your query is based on non-standard use of GROUP BY (I think SQLite allows that only for compatibility with MySQL) and I'm not at all sure that it will produce determinate results all the time.

Plus it uses MAX() on concatenated columns. Unless you somehow ensure that the two (concatenated) columns have fixed widths, the results will not be accurate for that reason as well.

I would write the query like this:

SELECT 
    m.message_timestamp, 
    m.message_id, 
    m.message_text,
    m.message_conversationId
FROM 
    ( SELECT message_conversationId         -- for every conversation
      FROM messages as m
      WHERE message_from <> 'me'            
      GROUP BY message_conversationId
    ) AS mc
  JOIN 
    messages AS m                           -- join to the messages
      ON  m.message_id =        
          ( SELECT mi.message_id            -- and find one message id
            FROM messages AS mi
            WHERE mi.message_conversationId      -- for that conversation
                  = mc.message_conversationId
              AND mi.message_from <> 'me'
            ORDER BY mi.message_timestamp DESC,  -- according to the
                     mi.message_id DESC          -- specified order
            LIMIT 1                              -- (this is the one part)
          ) ;