如果询问不同的值，MySQL group_concat 命令按 DESC 意外行为

Question

CREATE OR REPLACE TABLE `tmp` (
  `ord` int(5) unsigned NOT NULL DEFAULT 10000,
  `val` varchar(50) CHARACTER SET utf8 COLLATE utf8_estonian_ci NOT NULL DEFAULT ''
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `tmp` (`ord`, `val`)
VALUES (1,'foo'), (2,'baz'), (3,'baz'), (4,'foo');

Now consider the query

-- 1) Example with ASC sorting
select group_concat( distinct val 
ORDER BY ord  ASC SEPARATOR ';') from tmp;

As expected, it spits out foo;baz .
Question is, why

-- 2) Example with DESC sorting
select group_concat( distinct val 
ORDER BY ord DESC SEPARATOR ';') from tmp;

outputs baz;foo ?

It looks like if Mysql performed ascending sort and then just reversed it for DESC.
I would expect to return results from (4,'foo'), (3,'baz') .

Database version = 10.2.12-MariaDB-log

EDIT:

It's even more noticeable, if there is a little more data involved: DBFiddle

INSERT INTO `tmp` (`ord`, `val`)
VALUES (1,'foo'), (2,'baz'), (3,'baz'), (4,'bar'), (5,'foo');

select group_concat( distinct val 
ORDER BY ord ASC SEPARATOR ';') from tmp ;

--> foo;baz;bar

select group_concat( distinct val 
ORDER BY ord DESC SEPARATOR ';') from tmp;

--> bar;baz;foo ??
I really did expected foo;bar;baz here.

If DISTINCT is not required, database returns as expected no matter if ASC or DESC.

Answer 1

It looks like for your first example:

select group_concat( distinct val 
ORDER BY ord  ASC SEPARATOR ';') from tmp;

-- order
(1,'foo'), (2,'baz')

And it behaves according to order of retrieval 1 is before 2 so you get foo;bar .

In my opinion you need:

select group_concat( distinct val ORDER BY val  ASC SEPARATOR ';') from tmp;

DBFiddle Demo

---

EDIT:

This is the same situation like:

CREATE TABLE tmp(ord int,  val varchar(10)) ;
INSERT INTO tmp (ord, val) VALUES (1,'foo'), (2,'baz'), (3,'baz'), (4,'foo');

SELECT DISTINCT val
FROM tmp
ORDER BY ord;

DBFiddle Demo2

Please note that for other databases this query will return error:

PostgreSQL:
ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 3: ORDER BY ord;

SQL Server:
Msg 145 Level 15 State 1 Line 1
ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

Oracle:
ORA-01791: not a SELECTed expression

MySQL when SET @@sql_mode = 'ONLY_FULL_GROUP_BY';
ORDER BY clause is not in SELECT list, references column 'ord' 
which is not in SELECT list; this is incompatible with DISTINCT

More info: SELECT DISTINCT and ORDER BY in MySQL

---

Based on LISTAGG(DISTINCT ... ) from What's New in SQL:2016 :

listagg(distinct ...)

If ordered by the aggregated values: listagg(distinct X ,…) within group (order by X )

Answer 2

After investigating some more, it turns out quite simple and all returned results - expected or unexpected - are correct indeed.

The magical sentence in MySQL 5.7 Reference Manual is

Result set sorting occurs after values have been chosen

so

select group_concat( distinct val 
ORDER BY ord XXSC SEPARATOR ';') from tmp;

does the DISTINCT part of the job first and gets this as intermediate result:

-- unordered
(1,'foo'), (2,'baz')

Now engine takes this result and orders it. ASC or DESC as requested.