------------------------------------------------+
| 2017-01-04:3                                                |
+-------------------------------------------------------------+

NULLS:

CREATE OR REPLACE TABLE t1 (a int, b char);

INSERT INTO t1 VALUES (1, 'a'), (2, NULL);

SELECT GROUP_CONCAT(a, b) FROM t1;
+--------------------+
| GROUP_CONCAT(a, b) |
+--------------------+
| 1a                 |
+--------------------+

URL: https://mariadb.com/kb/en/group_concat/https://mariadb.com/kb/en/group_concat/