SQL Group By with limit and distinct - MySQL

Question

This is my tables and structure

Shoutbox

• SID //PK
• Title //Title

shoutbox_follower

• SID
• UID //User ID

Shouts

• SID
• Text //Shouts

I want to get the last 1 (latest) shouts(Shouts.Text) for each Shoutbox, which the user is following

---

i tried this but it did not worked

select shoutbox_follower.SID,shoutbox.title, shouts.text from shoutbox_follower, shoutbox, shouts where shoutbox_follower.uid=1;

But i can do the work with multiple query

SELECT SID from shoutBox_Follower where UID=5
SELECT SID,TEXT from Shouts where SID in ($boxList) order by id desc limit 1
SELECT Title from Shoutbox where SID=? limit 1

Answer 1

You can combine the queries, in cases like this:

SELECT SID,
       (select TEXT
       from Shouts
       where Shouts.SID = shoutBox_Follower.SID
       order by Shouts.SID desc limit 1
       ) as shout_text
from shoutBox_Follower
where UID=5

It performs very fast for small numbers of rows (assuming you've the needed indexes, of course). Based on your third query, the title can be fetched using a simple inner join.

Answer 2

In comments you mentioned that you need fast solution. If so - then add a field latest_shout_id to the shoutbox table and maintain it with AFTER INSERT trigger of shouts table. That's all.

Here is the query that will do what you want (assuming you have shout_id as a PK in shouts table and reference to shoutbox by shoutbox_id field):

    SELECT x.*
      FROM shoutbox_follower f
INNER JOIN (SELECT MAX(shout_id) shout_id,
                   shoutbox_id
              FROM Shouts s
          GROUP BY shoutbox_id) x ON x.shoutbox_id = f.sid 
     WHERE f.uid = 5

Changing it to correlated subquery may change it in faster or slower manner, it depends on a lot of factors.