Yak Shaving

This is needed when you need to concatenate multiple rows of a result set in MySQL.

CREATE TABLE services (
  id INT UNSIGNED NOT NULL,
  client_id INT UNSIGNED NOT NULL,
  KEY (id)
);

INSERT INTO services
VALUES (1,1),(1,2),(3,5),(3,6),(3,7);

SELECT id,client_id
       FROM services
       WHERE id = 3;

+----+-----------+
| id | client_id
+----+-----------+
|  3 |		 5
|  3 |		 6
|  3 |		 7
+----+-----------+

SELECT id,GROUP_CONCAT(client_id)
      FROM services
      WHERE id = 3 GROUP BY id;

+----+-------------------------+
| id | GROUP_CONCAT(client_id)
+----+-------------------------+
|  3 | 5,6,7
+----+-------------------------+

The group_concat function is affected by the group_concat_max_len and the max_allowed_packet variables. So, if you need to concatenate more than mysql can handle by default(1024 bytes) you just change the max_allowed_packet in my.cnf and set this:

SET [SESSION | GLOBAL] group_concat_max_len = VALUE;

into your session.