Select top 5 entries grouped by category

gistfile1.sql

SELECT *
FROM
(
    SELECT cp.cat_id, cp.entry_id, ct.title,
        @num := if(@group = cp.cat_id, @num + 1, 1) as row_number,
        @group := cp.cat_id as group_key
    FROM (SELECT @group:=null,@num:=0) n
    CROSS JOIN exp_category_posts cp
    LEFT JOIN exp_channel_titles ct on cp.entry_id = ct.entry_id
    ORDER BY cp.cat_id
) temp_table
WHERE row_number <= 5;