MySQL Bugs: #99485: Setting variable to CONCAT('') sets it to NULL instead of ''

Bug #99485	Setting variable to CONCAT('') sets it to NULL instead of ''
Submitted:	8 May 2020 9:08	Modified:	11 May 2020 18:11
Reporter:	Lars Tangvald	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Data Types	Severity:	S3 (Non-critical)
Version:	5.7.31	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
Original Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/mysql-5.7/+bug/1877442

Running CONCAT('') returns empty string, but setting a variable to the result of CONCAT('') sets it to NULL instead.

Works as expected in 8.0 and 5.7.29

How to repeat:
SELECT CONCAT('');
will result in an empty string as expected however if the result is assigned to a variable with:
SET @var = ""; SELECT @var := CONCAT('');
the variable will be NULL instead of an empty string.

Posted by developer:
 
Problem was introduced in 

commit d00361962f2836026c0661fb1ecb9067c5aed073 (HEAD)
Author: Maheedhar PV <maheedhar.panchalamarri.venka@oracle.com>
Date:   Wed Feb 5 11:32:05 2020 +0530

    Bug#30350206 - CONCAT() CAN GENERATE CORRUPTED OUTPUT

commit: d8d0c798e5

mysql> select concat('');
+------------+
| concat('') |
+------------+
|            |
+------------+
1 row in set (0,00 sec)

mysql> SET @var = ""; SELECT @var := CONCAT('');
Query OK, 0 rows affected (0,00 sec)

+--------------------+
| @var := CONCAT('') |
+--------------------+
|                    |
+--------------------+
1 row in set (0,00 sec)

commit: d00361962f2

mysql> select concat('');
+------------+
| concat('') |
+------------+
|            |
+------------+

1 row in set (0,00 sec)

mysql> SET @var = ""; SELECT @var := CONCAT('');
Query OK, 0 rows affected (0,00 sec)
+--------------------+
| @var := CONCAT('') |
+--------------------+
| NULL               |
+--------------------+
1 row in set (0,00 sec)

I will say this is critical. Our application, "slurm", crashes because of this. We cannot control jobs in our cluster.

Posted by developer:
 
Fixed in 5.7.31, 8.0.21.

Assigning CONCAT(') or CONCAT_WS(') to a variable set the variable
to NULL, not the empty string.

The bug is also present in 5.6.48 and will hopefully be fixed in 5.6.49