sqlpostgresqljoinrelational-divisionin-subquery

Postgresql: One-To-Many Relationship With Multiple Conditions On The Many Side


I have a Students table that has the following columns

id
student_name

And Courses table

id
student_id  //a foreign key
course_name
credits

It is a one to many relationship(one student might have multiple courses).

I want to list all the students that have these 2 courses

first course:   course_name -> math    , credit -> 5
second course:  course_name -> history , credit -> 3

Please note that each student has to have at least these two courses.

I can get what I want by joining with the Courses table twice(once for the first course and another for the second course), but in case I want to add one more course to the condition I will need to join one more time.

So can you please guide me to another approach.


Solution

  • Does this solve your problem? I count all occurrences for your expected courses and the sum must be 2.

    demo:db<>fiddle

    SELECT
        s.id
    FROM students s
    JOIN courses c
    ON c.student_id = s.id
    GROUP BY s.id
    HAVING SUM(
        ((c.course_name = 'math' AND c.credits = 5)
        OR
        (c.course_name = 'history' AND c.credits = 3))::int
    ) = 2
    

    Alternative to the SUM(condition::int) you could use the COUNT() with a FILTER clause:

    demo:db<>fiddle

    HAVING COUNT(*) FILTER (WHERE
        (c.course_name = 'math' AND c.credits = 5)
        OR
        (c.course_name = 'history' AND c.credits = 3)
    ) = 2