Maria DB GROUP BY id DESC with IN magic

Question

Tried get "ORDER BY id DESC" in "GROUP BY" to get only last comment. When i use "IN" with >1 elements its works fine, but with one element or without "IN". My MariaDB version is 10.0.36-MariaDB-0ubuntu0.16.04.1 Ubuntu 16.04.

Here is examples: 56xxx - true ids, 55xxx false ids

   MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid`='6576') GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
     WHERE (`commentsapi_comment`.`orderid` IN ('6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56218 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','-1')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56218 |
+-------+
1 row in set (0.01 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','6577')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56199 |
| 56218 |
+-------+
2 rows in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6577')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55813 |
+-------+
1 row in set (0.01 sec)

Who knows reason of this?

Answer 1

That's an invalid use of GROUP BY . Newer versions would spit at you.

When there are non-aggregate columns in the SELECT (eg, id ) that are not in the GROUP BY (which has only orderid ), the selected items are picked somewhat randomly.

Suggest you change id to

orderid, MIN(id), MAX(id), COUNT(*), GROUP_CONCAT(id)

IN is a red herring, not the cause.

If you need all the columns, not just id , you need to say so in the Question. There are many Q&A about "grouwise-max".