MySQL Count函数乘以两列的结果（应该分别返回每列的计数）

Question

I have a table of users, a table of tasks, and a table of reminders. I would like to return the count of tasks and the count of reminders per user. I can get it to work when I am only counting one or the other (Either reminders or tasks) but when I count both of them in one query they are for some reason multiplied by each other.

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

This is my query so far:

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

This is what my users table looks like:

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

This is what my tasks table looks like:

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

This is what my reminders table looks like:

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

I expect to get the following results from the above query:

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

I actually get the following:

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

Answer 1

Try below with distinct inside count: 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

You are getting a cross join, every reminder for every task.

try

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

Try With below query

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