MySQL Bugs: #79001: Wrong sort order with CONCAT and CASE-based sort column

Bug #79001	Wrong sort order with CONCAT and CASE-based sort column
Submitted:	28 Oct 2015 12:36	Modified:	31 Jan 2020 13:33
Reporter:	Matthias Pigulla	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: DML	Severity:	S3 (Non-critical)
Version:	5.0/5.1/5.5/5.6	OS:	Ubuntu
Assigned to:		CPU Architecture:	Any

Description:
The following query leads to a wrong result, the sort order is not applied correctly.

The result is
2-2015-10-15
2-2015-10-16
2-2015-10-15
but it should be
2-2015-10-16
2-2015-10-15
2-2015-10-15

How to repeat:
CREATE TABLE `test` (
  `test` date DEFAULT NULL
);
INSERT INTO `test` VALUES ('2015-10-15'),('2015-10-16'),('2015-10-15');
SELECT CONCAT(
  (CASE "foo" when "bar" then 1
   else 2 END) , "-",
  test
) AS bogus FROM test ORDER BY bogus DESC

C:\dbs>5.6\bin\mysql -uroot -p --port=3560  test
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.6.28 Source distribution PULL: 2015-OCT-24

Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> SELECT CONCAT(
    ->   (CASE "foo" when "bar" then 1
    ->    else 2 END) , "-",
    ->   test
    -> ) AS bogus FROM test ORDER BY bogus DESC;
+--------------+
| bogus        |
+--------------+
| 2-2015-10-15 |
| 2-2015-10-16 |
| 2-2015-10-15 |
+--------------+
3 rows in set (0.00 sec)

mysql> SELECT CONCAT(
    ->   2 , "-",
    ->   test
    -> ) AS bogus FROM test ORDER BY bogus DESC;
+--------------+
| bogus        |
+--------------+
| 2-2015-10-16 |
| 2-2015-10-15 |
| 2-2015-10-15 |
+--------------+
3 rows in set (0.00 sec)

Only repeatable on 5.0/5.1/5.5/5.6 and no anymore on 5.7. Thank you for the bug report.

C:\dbs>c:\dbs\5.7\bin\mysql -uroot -p --port=3570 --prompt="mysql 5.7 > "
Enter password: ******
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.10 Source distribution PULL: 2015-OCT-24

Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql 5.7 > use test
Database changed
mysql 5.7 > CREATE TABLE `test` (
    ->   `test` date DEFAULT NULL
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql 5.7 > INSERT INTO `test` VALUES ('2015-10-15'),('2015-10-16'),('2015-10-15');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql 5.7 > SELECT CONCAT(
    ->   (CASE "foo" when "bar" then 1
    ->    else 2 END) , "-",
    ->   test
    -> ) AS bogus FROM test ORDER BY bogus DESC;
+--------------+
| bogus        |
+--------------+
| 2-2015-10-16 |
| 2-2015-10-15 |
| 2-2015-10-15 |
+--------------+
3 rows in set (0.10 sec)

Can we expect a fix for <5.7, or is the impact not severe enough?

Fixed in 5.7.19.