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 makerar
GROUP BY cname) t
LEFT JOIN makerar m ON t.cname = m.cname AND t.max = m.avg;
参考链接
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