如何连接表并在mysql中定义更多条件

Question

I have two tables: courses and courses_partners . In my courses table I have attributes of each course. In courses_partners there are only two attributes:

course_id(foreign key to courses table)
partner_id(foreign key to partners table).

I want to select all courses , on which are joined exactly 8 partners(where num rows in courses_partners for one course_id is 8 (8 is number readed from courses.maxAmount attribute)). How to do it? I have something like this, but this not working:

SELECT courses_partners.course_id, courses.title, courses.maxAmount FROM 
courses_partners RIGHT JOIN courses ON 
(courses_partners.course_id=courses.course_id) GROUP BY
courses_partners.course_id HAVING COUNT(*) < courses.maxAmount

Answer 1

You can use:

SELECT *
FROM courses AS c
WHERE course_id IN (SELECT course_id
                    FROM courses_partners                    
                    GROUP BY course_id
                    HAVING COUNT(*) < c.maxAmount)

This will select all courses having less than maxAmount references to courses_partners table.

If you want those references to contain less than maxAmount records with distinct partner_id, then use this HAVING clause instead:

HAVING COUNT(DISTINCT partner_id) < c.maxAmount

Demo here

Alternatively you can use a LEFT JOIN :

SELECT c.*
FROM courses AS c
LEFT JOIN (
  SELECT course_id, COUNT(*) AS cnt
  FROM courses_partners                    
  GROUP BY course_id
) AS cp ON c.course_id = cp.course_id AND
           c.maxAmount < cp.cnt                    

This will also return courses having no referencing records in courses_partners at all.

Demo here