MySQL获得另一个表中所有行的值，该值等于当前表中的列数

Question

我正在用PHP编程一个论坛，所以我来到了需要计算线程数量的部分一个论坛。我使用下面的查询来获取所有论坛和各自的类别：

SELECT f.id 
    , f.name 
    , f.description 
    , c.id category_id 
    , c.name category_name 
    , c.description category_description 
    FROM forum_forums f 
    JOIN forum_forums_categories fc 
    ON f.id = fc.forum_id 
    JOIN forum_categories c 
    ON fc.category_id = c.id; 

它干得不错，我再能集团所有分类。接下来我想要做的事情是，将某个论坛中的线程数量添加到结果中的每一行，但我不确定如何执行此操作。

我有以下表格：forum_forums,forum_threads,forum_categories。此外，线程可以属于多个论坛（我有一个forums_threads_forums表，它将一个特定的thread_id绑定到forum_id）。

所以我的猜测是我需要在原始命令的某处添加一个计数。 此计数需要计算forum_threads_forums表中的行，其中forum_id等于它添加到结果中的当前论坛的行数。

---

为了让事情变得更简单，这里是（简化）的我想要达到一个例子： 表：forum_forums

id name 
1 forum1 
2 forum2 
3 forum3 

表：forum_threads

id title 
1 thread1 
2 thread2 

表：forum_threads_forums

thread_id forum_id 
1   1 
1   2 
2   1 
2   3 

然后我想查询返回（除其他事项外）：

forum_forums.id forum_forums.name forum.threads 
1    forum1    2 
2    forum2    1 
3    forum3    1 

如果有人可以把我在这将是伟大正确的方向。

编辑：

我想我可能需要一个子查询这样的SELECT COUNT(thread_id) AS thread_count FROM forum_threads_forums WHERE forum_id=:forum_id，但我不能确定在哪里放置这在我原来的查询

答：

以供将来参考，这里是我现在使用的工作命令：

SELECT 
    forum_forums.id, 
    forum_forums.name, 
    forum_forums.description, 
    COUNT(forum_threads_forums.thread_id) AS thread_count, 
    forum_categories.id AS category_id, 
    forum_categories.name AS category_name, 
    forum_categories.description AS category_description 
FROM 
    forum_forums 
LEFT OUTER JOIN 
    forum_threads_forums 
ON 
    forum_forums.id=forum_threads_forums.forum_id 
INNER JOIN 
    forum_forums_categories 
ON 
    forum_forums.id=forum_forums_categories.forum_id 
INNER JOIN 
    forum_categories 
ON 
    forum_forums_categories.category_id=forum_categories.id 
GROUP BY 
    forum_forums.id 

Answer 1

我怀疑有在聚集功能的入门级教程，不能覆盖这一点，但无论如何...

DROP TABLE IF EXISTS forums; 
CREATE TABLE forums 
(forum_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY 
,forum_name VARCHAR(12) NOT NULL UNIQUE 
); 


INSERT INTO forums VALUES 
(1 ,'forum1'),(2,'forum2'),(3,'forum3'); 

DROP TABLE IF EXISTS threads; 
CREATE TABLE threads 
(thread_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY 
,title VARCHAR(12) NOT NULL 
); 

INSERT INTO threads VALUES 
(1 ,'thread1'), 
(2 ,'thread2'); 

DROP TABLE IF EXISTS threads_forums; 
CREATE TABLE threads_forums 
(thread_id INT NOT NULL 
,forum_id INT NOT NULL 
,PRIMARY KEY(thread_id,forum_id) 
); 

INSERT INTO threads_forums VALUES 
(1   ,1), 
(1   ,2), 
(2   ,1), 
(2   ,3); 

SELECT f.* 
     , COUNT(t.thread_id) threads 
    FROM forums f 
    JOIN threads_forums tf 
    ON tf.forum_id = f.forum_id 
    JOIN threads t 
    ON t.thread_id = tf.thread_id 
    GROUP 
    BY forum_id; 
+----------+------------+---------+ 
| forum_id | forum_name | threads | 
+----------+------------+---------+ 
|  1 | forum1  |  2 | 
|  2 | forum2  |  1 | 
|  3 | forum3  |  1 | 
+----------+------------+---------+ 

注意，该解决方案将不会显示论坛，其中有没有线程。为此，您需要使用LEFT [OUTER] JOIN而不是

Answer 2

为了得到你想要的结果的简单的例子，运行

SELECT f.id, f.name, COUNT(*) AS threads 
FROM `forum_forums` f 
INNER JOIN `forum_threads_forums` r ON r.forum_id = f.id 
INNER JOIN `forum_threads` t ON t.id = r.thread_id 
GROUP BY f.id 
ORDER BY f.id 

Answer 3

在伪SQL：

SELECT f.id, f.name, COUNT(t.thread_id) FROM forum_threads_forums AS t 
JOIN forum_forums AS f ON t.forum_id=f.id 
GROUP BY t.forum_id 

Answer 4

使用您的简单表：

SELECT 
    forum_forums.id, 
    forum_forums.name, 
    COUNT(forum_threads_forums.thread_id) as threadCount 
FROM 
    forum_forums JOIN forum_threads_forums on forum_threads_forums.forum_id = forum_forums.id 
GROUP BY 
    forum_forums.id 

http://sqlfiddle.com/#!2/aeb7f/3

Answer 5

您可以使用LEFT JOIN和GROUP BY：

SELECT f.id 
     , f.name 
     , f.description 
     , c.id category_id 
     , c.name category_name 
     , c.description category_description, 
     , COUNT(ft.thread_id) as threads 
     FROM forum_forums f 
     LEFT JOIN forum_forums_categories fc 
     ON f.id = fc.forum_id 
     LEFT JOIN forum_categories c 
     ON fc.category_id = c.id 
     LEFT JOIN forum_threads_forums ft 
     ON f.id = ft.forum_id 
     GROUP BY f.id