正確的方法來加入表格和計數

Question

假設我有3個表格。 他們被稱爲用戶，線程，帖子。我需要知道用戶在一個sql行中創建了多少個&線程 。做這個的最好方式是什麼？加入我會做下面的事情，這是正確的/最好的方式來做到這一點？但是，我如何制定計數？

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

首先，你有流浪者逗號後SELECT *。其次，當你想要線程和帖子時，爲什麼要查詢用戶表？第三，爲什麼是（是嗎？）user_id a char？

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

如果您希望在一個查詢中使用，請使用UNION。

Answer 2

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 3

你可以使用這樣的事情，其應用和聚合函數來獲得總數：

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' 

如果你想分隔值，則：

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 4

將這工作？

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'

我不熱的SQL ...

Answer 5

你基本上沒有。因爲count是aggregate function，你可以簡單地組由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 

這樣，你的where條款變得可選。如果您想要所有用戶的明確列表及其相應的主題/帖子數，您可以將其刪除。