Question

我有两个表：

sales_header（包含一个created_at日期）
sales_line

我想计算每年/每月标题中的平均行数。

获取每月标头计数的基本查询很容易：

SELECT YEAR(h.created_at), MONTH(h.created_at), COUNT(h.id)
FROM   sales_header h
GROUP BY YEAR(h.created_at), MONTH(h.created_at)
ORDER BY YEAR(h.created_at), MONTH(h.created_at);

获得每个标题的平均行数也很容易：

SELECT ROUND(AVG(base.ChildCount), 0)
FROM (
  SELECT li.header_id AS HeaderId, COUNT(li.id) AS ChildCount
  FROM sales_header h
  JOIN sales_line li ON li.header_id = h.id
  GROUP BY li.header_id
) base;

我很难把两者放在一起。

谢谢！

更新

以下是一些示例数据，还有一些类似的内容：

CREATE TABLE sales_header (
  id INT NOT NULL,
  created_at TIMESTAMP NOT NULL,
  PRIMARY KEY (id)
);

CREATE TABLE sales_line (
  id INT NOT NULL AUTO_INCREMENT,
  header_id INT NOT NULL,
  PRIMARY KEY (id),
  FOREIGN KEY (header_id) REFERENCES sales_header(id)
);

INSERT INTO sales_header(id, created_at) VALUES
  (1, '2018-12-31T11:11:11'),
  (2, '2019-01-01T11:11:11'),
  (3, '2019-01-05T11:11:11'),
  (4, '2019-01-10T11:11:11'),
  (5, '2019-01-15T11:11:11'),
  (6, '2019-02-15T11:11:11'),
  (7, '2019-03-01T11:11:11'),
  (8, '2019-03-05T11:11:11'),
  (9, '2019-03-10T11:11:11')
;

INSERT INTO sales_line(header_id) VALUES
  (1), (1), (1), (1), (1), (1), (1), (1),
  (2), (2), (2), (2),
  (3), (3), (3), (3), (3), 
  (4), (4),
  (5),
  (6), (6), (6), (6), (6), (6),
  (7), (7),
  (8), (8),
  (9), (9), (9), (9), (9), (9), (9), (9), (9), (9), (9), (9), (9)
;

http://sqlfiddle.com/#!9/e33a94

我要出去的是这个

| Year | Month | Average Lines Per Header |
| ---- | ----- | ------------------------ |
| 2018 | 12    | 8                        |
| 2019 | 1     | 3                        |
| 2019 | 2     | 6                        |
| 2019 | 3     | 6                        |

Answer 1

将行数加入到标题表中。要获取行数，请按header_id和count汇总sales_line。

select year(h.created_at), month(h.created_at), count(*), avg(l.linecount)
from sales_header h
join
(
  select header_id, count(*) as linecount
  from sales_line
  group by header_id
) l on l.header_id = h.id
group by year(h.created_at), month(h.created_at)
order by year(h.created_at), month(h.created_at);

Answer 2

这项工作可以吗？

SELECT
    h.id
    YEAR(h.created_at),
    MONTH(h.created_at),
    COUNT(li.*)
FROM sales_header h
INNER JOIN sales_line li ON li.header_id = h.id
GROUP BY 1, 2, 3
ORDER BY h.id

每个月的平均子记录

2 个答案: