MySQL获取另一个表中所有行的计数，其中值等于当前表中的列

Question

我正在用PHP编写一个论坛，因此我需要计算一下论坛中的线程数量。 我正在使用以下查询来获取所有论坛及其各自的类别：

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;

它完成了工作，然后我可以将所有内容归类。 接下来，我还要将某个论坛中的线程数量添加到结果的每一行中，但我不确定该怎么做。

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

所以我的猜测是，我需要在原始命令中的某个地方添加一个计数。 此计数将需要计算forum_threads_forums表中的行，其中forum_id等于要添加到结果中的当前论坛的行数。

---

为了简化起见，下面是我要实现的一个示例（简化）：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

如果有人可以向正确的方向推动我，那将是很棒的。

编辑：

我认为我可能需要一个子查询，例如SELECT COUNT(thread_id) AS thread_count FROM forum_threads_forums WHERE forum_id=:forum_id但是我不确定将其放在我的原始查询中的位置

回答：

为了将来参考，这是我现在正在使用的工作命令：

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

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

Answer 2

我怀疑关于聚合函数的入门级教程无法涵盖这一点，但是无论如何...

 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 |
 +----------+------------+---------+

请注意，此解决方案将不会显示没有主题的论坛。 为此，您需要使用LEFT [OUTER] JOIN代替

Answer 3

要获得简化示例所需的结果，请运行

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 4

在伪SQL中：

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 5

使用更简单的表：

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