[#MDEV-25957] Order is not kept when using DISTINCT + ORDER BY in GROUP

The ORDER BY clause in the GROUP_CONCAT function allows you to specify the order, but if you add DISTINCT, the order will not be as expected.

MariaDB [relief]> select version();

+-------------------------------------+

| version()                           |

+-------------------------------------+

| 10.5.8-MariaDB-1:10.5.8+maria~focal |

+-------------------------------------+

1 row in set (0.000 sec)

MariaDB [relief]> CREATE OR REPLACE TABLE table1 ( id int auto_increment primary key, kind text, year int, ranking int );

Query OK, 0 rows affected (0.014 sec)

MariaDB [relief]> INSERT INTO table1 (kind, year, ranking) VALUES ('a', 2019, 1), ('a', 2018, 2), ('a', 2017, 1);

Query OK, 3 rows affected (0.007 sec)

Records: 3  Duplicates: 0  Warnings: 0

MariaDB [relief]> INSERT INTO table1 (kind, year, ranking) VALUES ('b', 2019, 6)

    -> ;

Query OK, 1 row affected (0.001 sec)

MariaDB [relief]> SELECT * FROM table1;

+----+------+------+---------+

| id | kind | year | ranking |

+----+------+------+---------+

|  1 | a    | 2019 |       1 |

|  2 | a    | 2018 |       2 |

|  3 | a    | 2017 |       1 |

|  4 | b    | 2019 |       6 |

+----+------+------+---------+

4 rows in set (0.000 sec)

Here, I concatted the `ranking` in the order of `year`.

MariaDB [relief]> SELECT GROUP_CONCAT(ranking ORDER BY year) FROM table1 GROUP BY kind;

+-------------------------------------+

| GROUP_CONCAT(ranking ORDER BY year) |

+-------------------------------------+

| 1,2,1                               |

| 6                                   |

+-------------------------------------+

2 rows in set (0.001 sec)

MariaDB [relief]>

Then I added DISTINCT.
The expected result is '1,2'.

MariaDB [relief]> SELECT GROUP_CONCAT(DISTINCT ranking ORDER BY year) FROM table1 GROUP BY kind;

+----------------------------------------------+

| GROUP_CONCAT(DISTINCT ranking ORDER BY year) |

+----------------------------------------------+

| 2,1                                          |

| 6                                            |

+----------------------------------------------+

2 rows in set (0.001 sec)

MariaDB [relief]>

I tried the same thing with MySQL 5.7, and got the same result.

Is it possible that the order is not kept when DISTINCT is added to the GROUP_CONCAT function?

[MDEV-25957] Order is not kept when using DISTINCT + ORDER BY in GROUP_CONCAT Created: 2021-06-18 Updated: 2021-06-18
Status:	Open
Project:	MariaDB Server
Component/s:	Data Definition - Procedure
Affects Version/s:	10.5.8
Fix Version/s:	None