MySQL ORDER BY AVG() DESC not working when certain columns are included

问题

I'm doing a query to return all the rows in table1, along with their average rating from table2:

SELECT `table1`.`description`, AVG( `table2`.`rating` ) AS avg_rating
FROM `table1` LEFT JOIN `table2` ON ( `table2`.`botid` = `table1`.`id` )
GROUP BY `table1`.`id`
ORDER BY avg_rating DESC

The problem is that even though I specify DESC, the results are being returned ASC:

+-------------+------------+
| description | avg_rating |
+-------------+------------+
| test2       |     1.0000 |
| test3       |     3.0000 |
| test4       |     3.0000 |
| saasdf      |     4.0000 |
+-------------+------------+

Why isn't MySQL honoring ORDER BY...DESC?

Even weirder, when I remove table1.description from the list of columns to retrieve, it works properly:

SELECT AVG( `table2`.`rating` ) AS avg_rating
FROM `table1` LEFT JOIN `table2` ON ( `table2`.`botid` = `table1`.`id` )
GROUP BY `table1`.`id`
ORDER BY avg_rating DESC

Returns:

+------------+
| avg_rating |
+------------+
|     4.0000 |
|     3.0000 |
|     3.0000 |
|     1.0000 |
+------------+

Here is my data:

table1:

id|description
--+-----------
 6|test2
16|test3
54|test4
72|saasdf

table2:

botid|rating
-----+------
    6|1
   16|3
   54|3
   72|4

(For the sake of this example there is a one-to-one relationship between the records in table1 and table2, but in reality there will be a one-to-many relationship.)

And my schema:

CREATE TABLE `table1` (
  `id` int(11) NOT NULL,
  `description` longtext NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `table2` (
  `botid` int(11) NOT NULL,
  `rating` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

There are indexes on both table1.id and table2.botid, although that shouldn't affect the results. I'm using MySQL 5.7.7-rc-log.

I have plenty of experience using aggregate functions, GROUP BY and ORDER BY but I've never come across anything like this. Any suggestions?

回答1:

Please upgrade to a GA version (5.7.9 was the first; 5.7.18 exists), then test again. IIRC, there was a bug somewhere in this area.

If the bug persists, provide the commands to reproduce the error and submit it to bugs.mysql.com .

I strongly recommend you change from MyISAM to InnoDB. Oracle may throw out the bug report since it involves MyISAM.

Meanwhile, you could see if this gives you the correct ordering:

SELECT  `table1`.`description`, 
        ( SELECT  AVG(`rating` )
            FROM  table2
            WHERE  botid = table1.id 
        ) AS avg_rating
    FROM  `table1`
    ORDER BY  avg_rating DESC

Provide EXPLAIN FORMAT=JSON SELECT ... for both your version and my version.

Explanation

Your original query appears to have the "inflate-deflate" problem of JOIN ... GROUP BY. First the JOIN gathers more "rows" than you started with, then the GROUP BY shrinks it back to the original number.

My rewrite sticks to the original number of rows (in table1) and probes table 2 for the necessary stuff. Primarily (in this situation) it avoids the tmp table and filesort.

回答2:

That weird behavior is because of longtext, If you can change your field type to CHAR/VARCHAR it will run perfectly. Or try something like:

SELECT CAST(table1.desc AS CHAR(128)) AS description, AVG( table2.rating ) AS avg_rating FROM table1 LEFT JOIN table2 ON ( table2.botid = table1.id ) GROUP BY table1.id ORDER BY avg_rating DESC;

回答3:

Working as expected for when run on maria db.Please check your query once because your order by is working on description column instead of avg_rating.

You can also try

SELECT `table1`.`description`, AVG( `table2`.`rating` ) AS avg_rating FROM `table1` LEFT JOIN `table2` ON ( `table2`.`botid` = `table1`.`id` ) GROUP BY `table1`.`id` ORDER BY 2 DESC;

and check the output again.will it remain the same?