既不能得到月份的总和，又不能得到每个月每天的平均总和

Question

我有一个表orders，我需要每个月获取order_sum的总和。使用组很容易。但是，然后我还需要获取每个月每天的平均金额。我正在使用联接，以便以后可以对结果进行排序。 （我正在使用MySQL版本5.6）

我遇到错误：

[on子句中的未知列'orders.month']

这是我的orders表：

+----+-----------+---------------------+
| id | order_sum | created_at          |
+----+-----------+---------------------+
| 1  | 25.13     | 2020-01-05 08:02:17 |
| 2  | 1.01      | 2020-01-25 20:13:20 |
| 3  | 5.04      | 2020-01-29 16:11:56 |
| 4  | 39.57     | 2020-02-17 15:14:09 |
| 5  | 63.01     | 2020-02-28 17:13:55 |
| 6  | 7         | 2020-03-02 07:10:02 |
+----+-----------+---------------------+

这是我的查询：

select
  MONTH(created_at) as month,
  sum(order_sum) as order_sum,
  second_table.avg_order_sum as average_per_day
from
  orders
inner join (
  select
    month(created_at) as month,
    avg(order_sum) as avg_order_sum
  from
    orders
) as second_table on orders.month = second_table.month
group by
  month;

这是我想要得到的结果

+-------+-----------+-----------------+
| month | order_sum | average_per_day |
+-------+-----------+-----------------+
| 1     | 31.18     | 10.39           |
| 2     | 102.58    | 51.29           |
| 3     | 7         | 7               |
+-------+-----------+-----------------+

我知道我可以使用此简单查询轻松获得每天的平均值，但无法使用内部联接将其合并为一个查询...

select month(created_at) as month, avg(order_sum) as average_per_day from orders group by month

这是架构

CREATE TABLE orders(
   id         INTEGER  NOT NULL PRIMARY KEY,
   order_sum  NUMERIC(11,2) NOT NULL,
   created_at VARCHAR(21) NOT NULL
);

INSERT INTO orders(id, order_sum, created_at) VALUES 
(1, 25.13, '2020-01-05 08:02:17'),
(2, 1.01, '2020-01-25 20:13:20'),
(3, 5.04, '2020-01-29 16:11:56'),
(4, 39.57, '2020-02-17 15:14:09'),
(5, 63.01, '2020-02-28 17:13:55'),
(6, 7.00, '2020-03-02 07:10:02');

[这里是有关架构的小提琴：http://sqlfiddle.com/#!9/b6b15/10

Answer 1

您需要在MONTH(created_at)和GROUP BY中引用JOIN ON（不是别名），并且还要对second_table进行分组。这对我有用：

select
  MONTH(created_at) as month,
  sum(order_sum) as order_sum,
  second_table.avg_order_sum as average_per_day
from
  orders
inner join (
  select
    month(created_at) as month,
    avg(order_sum) as avg_order_sum
  from
    orders
  group by month(created_at)
) as second_table on MONTH(created_at) = second_table.month
group by
  MONTH(created_at);

Answer 2

您可以简单地尝试：

SELECT month(created_at) AS month
    ,sum(order_sum) AS Order_Sum
    ,avg(order_sum) AS average_per_day
FROM orders
GROUP BY month