用平均列更新记录 [英] Updating a record with an average of a column

问题描述

我正在寻找status = 0的一列平均值，并将其更新到该表的另一条记录中. 这是我正在尝试使用的查询.

I'm looking to find the average of a column where status=0 and update it onto another record into that table. This is the query i'm attempting to use.

UPDATE mc25778 set balance=(AVG(balance WHERE status=0)) WHERE username="Average"

尝试执行此任务时出现此错误:

I get this error when trying to perform this task:

   Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE status=0)) WHERE username="Average"' at line 1

有什么办法可以解决这个问题吗?

Any ideas how I can sort this out?

谢谢！

推荐答案

您可以将表与子查询连接，该子查询分别计算每个用户名的平均值，

you can join the table with a subquery which separately calculate the average for every username,

UPDATE  mc25778 a
        INNER JOIN 
        (
            SELECT  username, AVG(balance) avg_bal
            FROM    mc25778 
            WHERE   status = 0
            GROUP   BY username
        ) b ON a.username = b.username
SET     a.balance = b.avg_bal
WHERE   a.username = 'Average'

更新1

您似乎想计算所有具有status = 0的记录的总平均值，其结果将在Average的记录上更新

It looks like you want to calculate the total average for all records having status = 0 and the result of it will be updated on the record of Average

UPDATE  mc25778 a
        CROSS JOIN
        (
            SELECT  AVG(balance) avg_bal 
            FROM    mc25778 
            WHERE   status = 0
        )  b
SET     a.balance = b.avg_bal
WHERE   a.username = 'Average'

• SQLFiddle演示
• SQLFiddle Demo

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