How to get x number of rows for each unique column value in MySQL?

Question

I have a very simple table and here is the values.

id      photo_name      user_id     created_at
----------------------------------------------
144     Photo1          1           2014-05-01 09:30:15
143     Photo2          2           2014-05-01 10:30:15
142     Photo3          1           2014-05-01 11:30:15
141     Photo4          3           2014-05-01 12:30:15
140     Photo5          1           2014-05-01 13:30:15
139     Photo6          2           2014-05-01 14:30:15
139     Photo6          2           2014-05-01 14:30:15

Expected result

I want to limit 2 photos per user_id

id      photo_name      user_id     created_at
----------------------------------------------
144     Photo1          1           2014-05-01 11:30:15
142     Photo3          1           2014-05-01 12:30:15
143     Photo2          2           2014-05-01 13:30:15
139     Photo6          2           2014-05-01 14:30:15
141     Photo4          3           2014-05-01 14:30:15

I've tried several ways but couldn't get the desired result. Thanks for help in advance!

Answer 1

The easiest way in MySQL is to use variables:

select id, photo_name, user_id, created_at
from (select p.*,
             @rn := if(@uid = user_id, @rn + 1, 1) as rn,
             @uid := user_id
      from photos p cross join
           (select @uid := -1, @rn := 0) var
      order by user_id
     ) p
where rn <= 2;

There is another way using standard SQL:

select p.*
from photos p
where 2 >= (select count(*)
            from photos p2
            where p2.user_id = p.user_id and
                  p2.id <= p.id
           );

What the subquery query does is count the number of photos by a given user. The p2.user_id = p.user_id restricts the query to just the user. The p2.id <= p.id puts in an ordering. So the lowest id for a user has only one match (itself) and the count is 1 . The next has two matches -- the lowest and itself -- so the value is 2. And so on.