UPDATE FROM 子句中的 GROUP BY [英] GROUP BY in UPDATE FROM clause

问题描述

我真的需要做这样的事情:

I really need do something like that:

UPDATE table t1 
SET column1=t2.column1 
FROM table t2 
INNER JOIN table t3 
USING (column2) 
GROUP BY t1.column2;

但是 postgres 说我有关于 GROUP BY 子句的语法错误.有什么不同的方法可以做到这一点?

But postgres is saying that I have syntax error about GROUP BY clause. What is a different way to do this?

推荐答案

UPDATE 语句不支持 GROUP BY，参见 文档.如果您尝试使用 t2 中的相应行更新 t1，您需要使用 WHERE 子句，如下所示:

The UPDATE statement does not support GROUP BY, see the documentation. If you're trying to update t1 with the corresponding row from t2, you'd want to use the WHERE clause something like this:

UPDATE table t1 SET column1=t2.column1
FROM   table t2
JOIN   table t3 USING (column2)
WHERE  t1.column2=t2.column2;

如果您需要在分配给 t1 之前对 t2/t3 中的行进行分组，则需要使用如下子查询:

If you need to group the rows from t2/t3 before assigning to t1, you'd need to use a subquery something like this:

UPDATE table t1 SET column1=sq.column1
FROM  (
   SELECT t2.column1, column2
   FROM   table t2
   JOIN   table t3 USING (column2)
   GROUP  BY column2
   ) AS sq
WHERE  t1.column2=sq.column2;

尽管按照公式，它不起作用，因为 t2.column1 不包含在 GROUP BY 语句中(它必须是一个聚合函数，而不是一个简单的列引用).

Although as formulated that won't work because t2.column1 isn't included in the GROUP BY statement (it would have to be an aggregate function rather than a simple column reference).

否则，你到底想在这里做什么?

Otherwise, what exactly are you trying to do here?

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