mysql> CREATE TABLE group_concat_test (
->id INT UNSIGNED NOT NULL auto_increment 
->PRIMARY KEY,
->_key INT UNSIGNED NOT NULL,
->_value CHAR(1) NOT NULL);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO group_concat_test
->(_key, _value) VALUES
->(1, 'b'),
->(1, 'c'),
->(2, 'a'),
->(2, 'd'),
->(2, 'c'),
->(3, 'b'),
->(3, 'e');
Query OK, 7 rows affected (0.01 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> SELECT _key,    
->COUNT(*) as cnt,
->GROUP_CONCAT(_value ORDER BY _value 
->SEPARATOR ', ')
->as value_list
->FROM group_concat_test
->GROUP BY _key
->ORDER BY _key;
+------+-----+------------+
| _key | cnt | value_list |
+------+-----+------------+
|    1 |   2 | b, c       |
|    2 |   3 | a, c, d    |
|    3 |   2 | b, e       |
+------+-----+------------+
3 rows in set (0.00 sec)