SQL with limit by result - sql

Question

I am making a simple chat, and need a way to get only 10 "messages" pr. conversation, but can't figure out how to do so.
This is what i already have tried.
my SQL:
SELECT c.* FROM chat c
INNER JOIN users su ON su.id = c.sender_user_id
INNER JOIN users ru ON ru.id = c.receiver_user_id
WHERE sender_user_id='1234' OR receiver_user_id='1234'
ORDER BY created ASC
Now i get ALL the messages for user ID 1234 ... BUT i only want 10 messages pr. conversation.
Hope it make sense

Answer 1

The standard SQL way is to use window/analytic functions:
SELECT t.*
FROM (SELECT c.*, su.name as sender_name, ru.name as receiver_name,
row_number() over (partition by c.conversation order by c.created desc) as seqnum
FROM chat c INNER JOIN
users su
ON su.id = c.sender_user_id INNER JOIN
users ru
ON ru.id = c.receiver_user_id
WHERE sender_user_id = '1234' OR receiver_user_id = '1234'
) t
WHERE seqnum <= 10
ORDER BY created ASC;
Note that you need to do a better job of selecting the columns you want in the subquery. Using * will result in duplicate column names.
EDIT:
In MySQL, you can do this using variables:
SELECT t.*
FROM (SELECT c.*, su.name as sender_name, ru.name as receiver_name,
(#rn := if(#c = c.conversation, #rn + 1,
if(#c := c.conversation, 1, 1)
)
) as seqnum
FROM chat c INNER JOIN
users su
ON su.id = c.sender_user_id INNER JOIN
users ru
ON ru.id = c.receiver_user_id CROSS JOIN
(SELECT #c := 0, #rn := 0) vars
WHERE sender_user_id = '1234' OR receiver_user_id = '1234'
ORDER BY c.conversation, c.created desc
) t
WHERE seqnum <= 10
ORDER BY created ASC;