DISTINCT and LAG window function

Question

There is simple table:

CREATE TABLE tab AS 
SELECT 1 AS t,   'G' AS company, 40 AS quote
UNION SELECT 2 , 'G',    60 
UNION SELECT 3 , 'S',    60 
UNION SELECT 4,  'S',    20;

And query:

SELECT DISTINCT company, quote ,LAG(quote) OVER(PARTITION BY company ORDER BY t)
FROM tab;

Output - result is consistent across RDBMSes.

+---------+-------+------+
| company | quote | lag  |
+---------+-------+------+
| G       |    40 | null |
| S       |    60 | null |
| G       |    60 | 40   |
| S       |    20 | 60   |
+---------+-------+------+

DBFiddle Demo - PostgreSQL

DBFiddle Demo - SQL Server

DBFiddle Demo - Oracle 11g

DBFiddle Demo - MariaDB

DBFiddle Demo - MySQL 8.0

But when I try to make some calculations I get different results:

SELECT DISTINCT company, quote - LAG(quote) OVER(PARTITION BY company ORDER BY t)
FROM tab;

PostgreSQL/SQL Server/Oracle (as I expected):

+---------+--------+
| company | result |
+---------+--------+
| G       | null   |
| G       | 20     |
| S       | null   |
| S       | -40    |
+---------+--------+

MariaDB/MySQL:

+----------+--------+
| company  | result |
+----------+--------+
| G        | null   |
| S        | null   |
+----------+--------+

Now as far as I know Logical Query Processing :

1. FROM
2. ON
3. JOIN
4. WHERE
5. GROUP BY
6. WITH CUBE/ROLLUP
7. HAVING
8. SELECT
9. DISTINCT

...

DISTINCT is after SELECT so the correct way should be like PostgreSQL/SQL Server/Oracle.(Yes I know that I could use subquery/remove DISTINCT but it is not the point of the question).

Is this behaviour a bug or is it working correctly(documentation)?

Answer 1

Definitely a bug in both MySQL and MariaDB. I've reported the issue to the two databases, linking back to this Stack Overflow question:

• https://bugs.mysql.com/bug.php?id=92503
• https://jira.mariadb.org/browse/MDEV-17242