Question

用户表：

+------------+---------+---------------------+
|    id      |  name   |created_at           |
+------------+---------+---------------------+
| 1          | AAA     | 2013-01-10 01:30:00 |             
| 2          | BBB     | 2013-02-14 01:30:00 |
| 3          | CCC     | 2013-03-29 01:30:00 |
| 4          | DDD     | 2013-04-30 01:30:00 |
| 5          | EEE     | 2013-05-10 01:30:00 |
| 6          | FFF     | 2013-06-10 01:30:00 |
| 7          | GGG     | 2013-07-10 01:30:00 |
| 8          | IIS     | 2013-08-10 01:30:00 |
| 9          | IIE     | 2013-09-10 01:30:00 |
| 10         | IIF     | 2013-10-10 01:30:00 |
| 11         | IIG     | 2013-11-10 01:30:00 |
| 12         | IIH     | 2013-12-18 01:30:00 |
| 13         | IHH     | 2013-10-18 01:30:00 |
| 14         | AHH     | 2013-02-18 01:30:00 |
| 15         | AEH     | 2013-02-18 01:30:00 |
+------------+---------+---------------------+

帖子表：

+------------+-------------------+---------------------+
|    id      |  user_id          | created_at          |
| 1          | 1                 | 2013-02-10 01:30:00 |
| 2          | 2                 | 2013-03-10 01:30:00 |
| 3          | 2                 | 2013-03-10 01:30:00 |
| 4          | 7                 | 2013-04-10 01:30:00 |
| 5          | 1                 | 2013-05-10 01:30:00 |
| 6          | 8                 | 2013-05-10 01:30:00 |
| 7          | 3                 | 2013-06-10 01:30:00 |
| 8          | 6                 | 2013-07-10 01:30:00 |
| 9          | 7                 | 2013-08-10 01:30:00 |
| 10         | 9                 | 2013-09-10 01:30:00 |
| 11         | 11                | 2013-10-10 01:30:00 |
| 12         | 4                 | 2013-11-10 01:30:00 |
+------------+-------------------+---------------------+

从上表中，我如何能够找出上一年每个用户的平均帖子数量。请注意：1月和12月没有帖子。因此，那些月平均值为0.

临时表（不存在，仅供参考）：每月总帖子数和累计用户数：

上一年度的每月累计用户数（累计）：

+--------------+-------------------+
|    month     |  total_users(cumulative)      
| Jan          | 1                 |
| Feb          | 4                 |
| Mar          | 5                 |
| Apr          | 6                 |
| May          | 7                 |
| Jun          | 8                 |
| Jul          | 9                 |
| Aug          | 10                |
| Sep          | 11                |
| Oct          | 13                |
| Nov          | 14                |
| Dec          | 15                |
+--------------+-------------------+

每月total_posts（去年）：

+--------------+-------------------+
|    month     |  total_posts      |
| Jan          | 0                 |
| Feb          | 1                 |
| Mar          | 2                 |
| Apr          | 1                 |
| May          | 2                 |
| Jun          | 1                 |
| Jul          | 1                 |
| Aug          | 1                 |
| Sep          | 1                 |
| Oct          | 1                 |
| Nov          | 1                 |
| Dec          | 0                 |
+--------------+-------------------+

等式：

average_posts_per_user：

 total_posts_per_month / total_users_per_month (cumulative)

预期结果：

+--------------+-------------------+
|    month     |  avg_posts        |
| Jan          | 0                 |
| Feb          | 0.25              |
| Mar          | 0.4               |
| Apr          | 0.16              |
| May          | 0.28              |
| Jun          | 0.12              |
| Jul          | 0.11              |
| Aug          | 0.1               |
| Sep          | 0.09              |
| Oct          | 0.8               |
| Nov          | 0.7               |
| Dec          | 0                 |
+--------------+-------------------+

先谢谢。

Answer 1

如果您不需要平均值为0的行

SELECT
    SUBSTRING(created_at,1,7) as `month`,
    (COUNT(DISTINCT id) / COUNT(DISTINCT user_id)) as `average`
FROM
    posts 
WHERE
    YEAR(created_at) = '2013'
GROUP BY
    SUBSTRING(created_at,1,7)

如果您需要平均值为0的行

SELECT
    tmonths.`month`,
    (COUNT(DISTINCT posts.id) / COUNT(DISTINCT posts.user_id)) as `average`
FROM
    (
        SELECT '2013-01' as `month`
        UNION SELECT '2013-02' as `month`
        UNION SELECT '2013-03' as `month`
        UNION SELECT '2013-04' as `month`
        UNION SELECT '2013-05' as `month`
        UNION SELECT '2013-06' as `month`
        UNION SELECT '2013-07' as `month`
        UNION SELECT '2013-08' as `month`
        UNION SELECT '2013-09' as `month`
        UNION SELECT '2013-10' as `month`
        UNION SELECT '2013-11' as `month`
        UNION SELECT '2013-12' as `month`
    ) tmonths
LEFT JOIN
    posts ON SUBSTRING(posts.created_at,1,7) = tmonths.`month`
GROUP BY
    tmonths.`month`

Answer 2

怎么样？

SELECT
  userqry.`month`,
  total_users,
  IFNULL(month_posts,0) AS total_posts,
  IFNULL(month_posts,0)/total_users AS avg_posts
FROM
  (SELECT
    `month`,
    @usernum:=@usernum+new_users AS total_users
  FROM (
    SELECT
      DATE_FORMAT(created_at,'%M') AS `month`,
      COUNT(id) AS new_users
    FROM users
    GROUP BY 
      MONTH(created_at)
  ) AS baseview,
  (SELECT @usernum:=0) AS usernuminit
  ) AS userqry
LEFT JOIN
  (SELECT
    DATE_FORMAT(created_at,'%M') AS `month`,
    COUNT(id) AS month_posts
  FROM posts
  GROUP BY 
    MONTH(created_at)
  ) AS postqry
ON userqry.`month`=postqry.`month`

SQLfiddle

修改

年度比较使用：

SELECT 2013 AS reportyear, view2013.* FROM ( -- query from above with "WHERE created_at BETWEEN '2013-01-01' AND '2013-12-31'" in both inner queries ) AS view2013 UNION SELECT 2014 AS reportyear, view2014.* FROM ( -- query from above with "WHERE created_at BETWEEN '2014-01-01' AND '2014-12-31'" in both inner queries ) AS view2015

或（推荐）从您的应用程序多次运行上述查询，每次都有不同的年份。

上一年度每个用户的平均帖子 - MYSQL

2 个答案: