What is the best way to select the first two records of each group by a “SELECT” command?

Question

For instance I have the following table:

id group data
1 1 aaa
2 1 aaa
3 2 aaa
4 2 aaa
5 2 aaa
6 3 aaa
7 3 aaa
8 3 aaa

What is the best way to

Answer 1

I like this trick, that makes use of GROUP_CONCAT aggregate function, and FIND_IN_SET:

SELECT
  Tablename.*
FROM
  Tablename INNER JOIN (
    SELECT `group`, GROUP_CONCAT(id ORDER BY id) ids
    FROM Tablename
    GROUP BY `group`) grp ON
  Tablename.`group` = grp.`group` AND
  FIND_IN_SET(Tablename.id, ids)<=2
ORDER BY
  Tablename.`group`, Tablename.id

Performances can't be too good, as it can't make use of an index.

Or you can also use this:

SELECT t1.id, t1.`group`, t1.data
from
  Tablename t1 INNER JOIN Tablename t2
  ON t1.`group` = t2.`group` AND t1.id>=t2.id
GROUP BY
  t1.id, t1.`group`, t1.data
HAVING
  COUNT(*)<=2
ORDER BY
  t1.`group`, t1.id, t1.data