MySQL get count of all rows in another table where the value equals column in current table

Question

I'm programming a forum in PHP, and so I've come to the part where I'll need to count the amount of threads in a forum. I'm using the following query to get all forums and their respective categories:

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;

It gets the job done and I'm then able to group everything into categories. What I want to do next, is also add the amount of threads that are in a certain forum to each row in the results, and I'm unsure how to do that.

I have the following tables: forum_forums , forum_threads , forum_categories . Also, threads can belong to multiple forums (I have a forums_threads_forums table which binds a specific thread_id to a forum_id ).

So my guess is I would need to add a count in the original command somewhere. This count would need to count the rows in the forum_threads_forums table where forum_id is equal to that of the current forum it is adding to the results.

---

To make things simpler, here is an example of what I'm trying to achieve (simplified): Table: forum_forums

id  name
1   forum1
2   forum2
3   forum3

Table: forum_threads

id  title
1   thread1
2   thread2

Table: forum_threads_forums

thread_id  forum_id
1          1
1          2
2          1
2          3

Then I would like the query to return (amongst other things):

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

If anyone could push me in the right direction that would be great.

EDIT:

I think I might need a subquery such a SELECT COUNT(thread_id) AS thread_count FROM forum_threads_forums WHERE forum_id=:forum_id but I'm unsure where to place this in my original query

ANSWER:

For future reference, here is the working command I'm using now:

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

You can use LEFT JOIN and 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

I doubt that there's an entry level tutorial on aggregate functions that fails to cover this, but anyway...

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

Note that this solution will not show forums for which there are no threads. For that, you'd need to use a LEFT [OUTER] JOIN instead

Answer 3

To get your desired result for the simplified example, run

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

In pseudo 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 5

Using your simpler table:

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