mysql - MySQL 中的非聚合平均值？

Question

有没有一种方法可以在不使用聚合函数的情况下计算子查询返回结果的平均值？

假设的例子:

我想为每个帐户选择一个月内的平均最大发票数。我希望能够做这样的事情:

SELECT
   accounts.*,
   **AVERAGE**(SELECT MAX(`amount`)
               FROM invoices
               WHERE invoices.account_id = accounts.id
               GROUP BY invoices.month) as `average_max`

Answer 1

根据定义，平均是一种聚合函数，因为它需要对数据的聚合进行操作。

为了得到你想要的，我会这样做:

SELECT
  accounts.*, avg_amount.amount AS avg_amount
FROM
  accounts
LEFT JOIN (
  SELECT account_id, AVG(amount) AS amount FROM (
    SELECT month, account_id, MAX(amount) AS amount FROM invoices GROUP BY month, account_id
  ) AS max_amount USING(account_id)
) AS avg_amount ON (accounts.id = avg_amount.account_ID)