Question

假设我有3张桌子。它们被称为用户，线程，帖子。我需要知道有多少帖子＆amp;线程用户已在一个sql行中创建。做这个的最好方式是什么？加入我会做类似跟随的事情，这是正确/最好的方法吗？但我如何计算？

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

你可以使用类似这样的东西来应用和聚合函数来得到总数：

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，你在SELECT *后有逗号逗号。第二，当你想要线程和帖子时，为什么查询用户表？第三，为什么（是吗？）user_id一个char？

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

如果您想在一个查询中使用UNION，请使用

Answer 3

你基本上就在那里。由于count是aggregate function，因此您只需按user_id进行分组：

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子句变为可选。如果您想要一个具有相应线程/帖子数的所有用户的明确列表，您可以将其删除。

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

这会有用吗？

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'

我不会热衷于SQL ......

正确的连接表和计数的方法

5 个答案: