Array slicing / group_concat limiting in MySQL

Question

Let's say I have a table:

i j
---
a 1
a 2
a 3
a 4
b 5
b 6
b 7
b 8
b 9

Obvoiusly SELECT a, GROUP_CONCAT(b SEPARATOR ',') GROUP BY a would give me

a  1,2,3,4
b  5,6,7,8,9

But what if I want to get only a LIMITED number of results, like 2, for example:

a  1,2
b  5,6

Any ideas?

Answer 1

Best way is to use SUBSTRING_INDEX() and GROUP_CONCAT().

SELECT i, SUBSTRING_INDEX( GROUP_CONCAT(j), ',', 2)
FROM mytable
GROUP BY i;

You don't have to know the length of j field here.

Answer 2

One solution would be to limit the rows in the group to the top two, before computing the aggregate.

SELECT t.i, GROUP_CONCAT(t.j)
FROM
  (SELECT t1.i, t1.j
   FROM mytable AS t1
     LEFT JOIN mytable AS t2
       ON (t1.i = t2.i AND t1.j >= t2.j) 
   GROUP BY t1.i, t1.j
   HAVING COUNT(*) <= 2) AS t
GROUP BY t.i;

Another solution, if you know the values in j are of a fixed length, is to simply use SUBSTRING() on the result:

SELECT i, SUBSTRING( GROUP_CONCAT(j), 1, 3 )
FROM mytable
GROUP BY i;