mysql - 连接表和计数的正确方法

Question

假设我有 3 张桌子。它们被称为用户、线程、帖子。我需要知道用户在一个 sql 行中创建了多少帖子和线程。做这个的最好方式是什么？我会执行以下操作的连接，这是正确的/最好的方法吗？但是我该如何计算呢？

SELECT *,
FROM users
LEFT JOIN threads on threads.user_id=users.user_id
LEFT JOIN posts on posts.user_id=users.user_id
WHERE user_id='1'

Answer 1

您可以使用类似这样的应用和聚合函数来获取总数：

SELECT u.user_id, 
    coalesce(TotalThreads, 0) + coalesce(TotalPosts, 0) Total
FROM users u
LEFT JOIN
(
  select user_id, count(thread_id) TotalThreads
  from threads
  group by user_id
) t
  on t.user_id=u.user_id
LEFT JOIN
(
  select user_id, count(post_id) TotalPosts
  from posts
  group by user_id
) p
  on p.user_id=u.user_id
WHERE u.user_id='1'

如果您希望将值分开，则：

SELECT u.user_id, 
   coalesce(TotalThreads, 0) TotalThreads, 
   coalesce(TotalPosts, 0) TotalPosts
FROM users u
LEFT JOIN
(
  select user_id, count(thread_id) TotalThreads
  from threads
  group by user_id
) t
  on t.user_id=u.user_id
LEFT JOIN
(
  select user_id, count(post_id) TotalPosts
  from posts
  group by user_id
) p
  on p.user_id=u.user_id
WHERE u.user_id='1'

Answer 2

首先，您在 . 之后有杂散逗号SELECT *。第二，当你想要线程和帖子时，为什么要查询用户表？第三，为什么（是？）user_ida char？

SELECT COUNT(user_id) FROM threads WHERE user_id='1'
SELECT COUNT(user_id) FROM posts WHERE user_id='1'

UNION如果您想在一个查询中使用它。

Answer 3

你基本上在那里。因为count是一个聚合函数，您可以简单地按以下方式分组user_id：

select users.user_id as [User ID]
,count(threads.user_id) as [Number of Threads]
,count(posts.user_id) as [Number of Posts]
from users
left join threads on threads.user_id = users.user_id
left join posts on posts.user_id = users.user_id
where user_id = '1'
group by users.user_id

这样，您的where子句就变成了可选的。如果您想要所有用户的不同列表及其相应的线程/帖子计数，则可以将其删除。

Answer 4

    SELECT 
    COUNT() as posts,
    FROM users
    LEFT JOIN threads on threads.user_id=users.user_id
    LEFT JOIN posts on posts.user_id=users.user_id
    WHERE user_id='1'

Answer 5

这行得通吗？

SELECT COUNT (*) FROM users u, thread t, posts p WHERE t.user_id=u.user_id AND p.user_id=u.user_id AND user_id='1'

我对 SQL 不是很感兴趣……