无法在MySQL中使用某些GROUP BY条件对AVG值进行ORDER BY [英] Can not ORDER BY an AVG value with certain GROUP BY criteria in MySQL

问题描述

我有一张桌子data_summaries.它具有诸如item_id INT(11)，user_grouping TEXT和value DECIMAL(10,2)的列.

I have a table data_summaries. It has such columns as item_id INT(11), user_grouping TEXT and value DECIMAL(10,2).

如果我尝试进行按user_grouping分组结果并按value的AVG排序结果的查询，将失败:

If I try to make a query that groups the results by user_grouping and orders them by the AVG of value, that fails:

SELECT user_grouping, AVG(value) AS avg_value, SUM(value) AS sum_value
FROM data_summaries
GROUP BY user_grouping
ORDER BY avg_value
+---------------+-----------+-----------+
| user_grouping | avg_value | sum_value |
+---------------+-----------+-----------+
| London        | 50.609733 |  18978.65 |
| Paris         | 50.791733 |  19046.90 |
| New York      | 51.500400 |   2575.02 |
| NULL          | 49.775627 |  18665.86 |
+---------------+-----------+-----------+

ORDER BY子句确实在做某事，因为它确实改变了顺序:

The ORDER BY clause does seem to be doing something as it does change the order:

SELECT user_grouping, AVG(value) AS avg_value, SUM(value) AS sum_value
FROM data_summaries
GROUP BY user_grouping
+---------------+-----------+-----------+
| user_grouping | avg_value | sum_value |
+---------------+-----------+-----------+
| NULL          | 49.775627 |  18665.86 |
| New York      | 51.500400 |   2575.02 |
| London        | 50.609733 |  18978.65 |
| Paris         | 50.791733 |  19046.90 |
+---------------+-----------+-----------+

另一方面，按value的SUM排序可以按预期工作:

On the other hand, ordering by the SUM of value works as expected:

SELECT user_grouping, AVG(value) AS avg_value, SUM(value) AS sum_value
FROM data_summaries
GROUP BY user_grouping
ORDER BY sum_value
+---------------+-----------+-----------+
| user_grouping | avg_value | sum_value |
+---------------+-----------+-----------+
| New York      | 51.500400 |   2575.02 |
| NULL          | 49.775627 |  18665.86 |
| London        | 50.609733 |  18978.65 |
| Paris         | 50.791733 |  19046.90 |
+---------------+-----------+-----------+

如果我改为按item_id分组，则按AVG进行排序是可行的:

If I group by item_id instead, then ordering by the AVG works:

SELECT item_id, AVG(value) AS avg_value, SUM(value) AS sum_value
FROM data_summaries
GROUP BY item_id
+---------+-----------+-----------+
| item_id | avg_value | sum_value |
+---------+-----------+-----------+
|       4 | 49.318225 |  11392.51 |
|       1 | 49.737835 |  11489.44 |
|       2 | 50.420606 |  11647.16 |
|       6 | 51.024242 |  11786.60 |
|       5 | 51.456537 |  11886.46 |
|       3 | 53.213000 |   1064.26 |
+---------+-----------+-----------+

我该如何更改第一个查询以使其按平均值排序?

How would I need to change the first query to get it ordered by the average?

推荐答案

这是一个MySQL错误，请参见

That is a MySQL bug, see Unexpected order for grouped query, that involves avg() in combination with grouping by a text-column. It is still open in MySQL 5.7.15.

作为一种解决方法，您可以将数据类型更改为例如varchar.如果您不需要索引来加快速度，则投射也应该起作用:

As a workaround, you can change your datatype to e.g. varchar. If you don't need indexes to speed it up, casting should work too:

SELECT cast(user_grouping as char(200)), AVG(value) AS avg_value, SUM(value) AS sum_value
FROM data_summaries
GROUP BY cast(user_grouping as char(200))
ORDER BY avg_value

更新:

该错误已在中修复. MySQL 5.7.17 :

在基于BLOB的类型的列上进行分组并根据AVG()，VAR_POP()或STDDEV_POP()聚合函数的结果进行排序的查询，如果InnoDB是临时的，则以错误的顺序返回结果表被使用. (缺陷号22275357，缺陷号79366)

Queries that were grouped on a column of a BLOB-based type, and that were ordered on the result of the AVG(), VAR_POP(), or STDDEV_POP() aggregate function, returned results in the wrong order if InnoDB temporary tables were used. (Bug #22275357, Bug #79366)

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