MySQL Count函數乘以兩列的結果（應該分別返回每列的計數）

Question

我有一個用戶表，一個任務表和一個提醒表。 我想返回任務計數和每個用戶的提醒計數。 當我只計算一個或另一個（提醒或任務）時，我可以讓它工作但當我在一個查詢中計算它們時，由於某種原因它們相互成倍增加。

SQLFiddle： http ：//www.sqlfiddle.com/#！9 / f0d6696 / 1/0

這是我目前的查詢：

SELECT
  users.name,
  COUNT(reminders.id),
  COUNT(tasks.id)
FROM users
  LEFT JOIN reminders on users.id = reminders.id
  LEFT JOIN tasks on users.id = tasks.id
GROUP BY users.id

這是我的用戶表的樣子：

+---------------------------------------+
| ID | Name       | Email               |
+---------------------------------------+
| 1  | John Smith | jsmith@email.com    |
| 2  | Mark Twain | mtwain@books.com    |
| 3  | Elon Musk  | space-dude@email.com|
+---------------------------------------+

這是我的任務表的樣子：

+------------------------------------------------+
| ID | Title       | Text            | Status    |
+------------------------------------------------+
| 1  | Dishes      | Kitchen = nasty | incomplete|
| 1  | Library     | drop off books  | complete  |
| 3  | Gym         | get swole dude  | incomplete|
+------------------------------------------------+

這是我的提醒表的樣子：

+------------------------------------+
| ID | Title       | Text            | 
+------------------------------------+
| 1  | Dishes      | Kitchen = nasty |
| 2  | Library     | drop off books  |
| 1  | Gym         | get swole dude  |
+------------------------------------+

我希望從以上查詢中獲得以下結果：

+-------------------------------------------+
| Name             | Tasks   | Reminders    |
+-------------------------------------------+
| John Smith       |    2    |      2       |
| Mark Twain       |    1    |      0       |
| Elon Musk        |    0    |      1       |
+-------------------------------------------+

我實際上得到以下內容：

+-------------------------------------------+
| Name             | Tasks   | Reminders    |
+-------------------------------------------+
| John Smith       |    4    |      4       |   <---2 tasks x 2 reminders?
| Mark Twain       |    1    |      0       |
| Elon Musk        |    0    |      1       |
+-------------------------------------------+

Answer 1

請嘗試下面的內部計數： http ： //www.sqlfiddle.com/#！9 / f0d6696 / 12

  SELECT
  users.name,

  count(distinct reminders.title) as rtitle,

  count(distinct tasks.title) as ttitle
FROM users
  LEFT JOIN reminders on users.id = reminders.id
  LEFT JOIN tasks on users.id = tasks.id
  group by users.name

Answer 2

您正在獲得交叉加入，每個任務的每個提醒。

嘗試

select 
users.name,
remindercount,
taskcount
FROM users
LEFT JOIN (select id, count(*) as remindercount from reminders group by id) reminders on users.id = reminders.id
LEFT JOIN (select id, count(*) as taskcount from tasks group by id) tasks on users.id = tasks.id

Answer 3

嘗試使用以下查詢

SELECT id，email， name ，（SELECT COUNT（id）FROM reminders r WHERE r.id = u.id）AS提醒，（SELECT COUNT（id）FROM tasks t WHERE t.id = u.id）AS task FROM users ü