mysql limit inside group?

Question

I want to limit the size of records inside a group, and here is my trial, how to do it right?

mysql> select * from accounts limit 5 group by type;
         


        

Answer 1

This will probably do the trick, although if type isn't indexed, it'll be sloooowwww. And even with one, it's not especially fast:

SELECT a.*
FROM accounts a
     LEFT JOIN accounts a2 ON (a2.type = a.type AND a2.id < a.id)
WHERE count(a2.id) < 5
GROUP BY a.id;

A better bet would be to just order the list by type and then use a loop at the business layer to remove the rows you don't want.

Answer 2

Group by is used for aggregate functions (sums, averages...)

Is allows you to split the aggregate result into groups. You have not used one of these functions.