must appear in the GROUP BY clause or be used in an aggregate function

今天在分组统计的时候pgsql报错 must appear in the GROUP BY clause or be used in an aggregate function，在mysql里面是可以的，但是pgsql报错，我去stackoverflow查询了一下，发现有人遇到过和我一样的问题，这是pgsql一个常见的聚合问题，在SQL3标准以前，选择显示的字段必须出现在在 GROUP BY 中。下面我把问题描述一下：

有一张表叫 makerar，表中记录如下：
cname | wmname | avg
--------|--------|-----------------------
canada | zoro | 2.00
spain | luffy | 1.00
spain | usopp | 5.00

我想要查询每个 cname 的最大 avg，按照mysql的写法是

SELECT cname, wmname, MAX(avg) FROM makerar GROUP BY cname;

在pgsql中报错

ERROR:  column "makerar.wmname" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT cname, wmname, MAX(avg)  FROM makerar GROUP BY cname;

意思是 wmname 字段必须在 GROUP BY 中出现或者被用于聚合函数

于是我按照错误提示，把 wmname 字段加在 GROUP BY 后面，即

SELECT cname, wmname, MAX(avg) FROM makerar GROUP BY cname, wmname;

得到的结果是

cname	wmname	avg
canada	zoro	2.00
spain	luffy	1.00
spain	usopp	5.00

而我期望得到的结果是

cname	wmname	avg
canada	zoro	2.00
spain	usopp	5.00

解决方案有两种，但是我只看懂了一种，于是把这一种记录一下

大体思路是在子查询中完成聚合，然后关联包含你想显示字段的表（这里是makerar自身）获取字段（这里是wmname），所以sql就变成了下面这个样子

SELECT   t.cname,m.wmname,t.max
FROM (SELECT cname,MAX(avg) AS max FROM makerarGROUP BY cname) t
LEFT JOIN makerar m ON t.cname = m.cname AND t.max = m.avg;

参考链接

https://stackoverflow.com/questions/19601948/must-appear-in-the-group-by-clause-or-be-used-in-an-aggregate-function#

转载于:https://www.cnblogs.com/zxhyJack/p/9934259.html

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must appear in the GROUP BY clause or be used in an aggregate function

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