Question

I am working on a MYSQL, and I am looking for a solution to show data grouping by ratio.

I have a COURSE table which contain courses id and courses ratio. I have another table, ASK, which contain askers id, and another field course id pointing on COURSE table (ids correspond). With one SQL query, I would like to display all asks grouping by ratio and rejecting other demands. Demands are considered by seniority list.

By example, I have 3 courses: the first one allow only 2 people and 2 other courses allow only one person. In the ASK table, we have 3 askers (JACK, JOE AND JOHN) pointing on first course, (JOHN AND JOE) 2 on second and third (JACK AND JOHN). JACK is the older, after its JOHN and after its JOE.

I would like to see:

course id   studentName 
-------------------------
1           JACK         
1           JOHN            
2           JOHN
3           JACK

Answer 1

SELECT c.courseID
       s.studentName
FROM course AS c
    JOIN asks AS a
        ON a.courseID = c.courseID
    JOIN student AS s
        ON s.studentName = a.studentName
    JOIN asks AS a2
        ON a2.courseID = c.courseID
    JOIN student AS s2
        ON s2.studentName = a2.studentName
        AND s2.seniority <= s.seniority
GROUP BY c.courseID
       , c.ratio
       , s.studentName
HAVING c.ratio >= COUNT(*)
ORDER BY c.courseID
       , s.seniority