SQL - How to list items which are below the average - mysql

Question

I have quite a basic databast of 3 tables. "Students" "Tests" and "Scores"
For each test I need to list all students with test scores that are below the average for that test. (If that makes any sense at all)
I have an SQL query which simply prints the average score for each test.
SELECT t.Test_name, AVG(sc.Result) AS Avgscore
FROM Tests t
JOIN Scores sc ON t.id_Tests = sc.Tests_id_Tests
JOIN Students s ON sc.Students_id_Students = s.id_Students
WHERE t.id_Tests = $c"
($c is a parameter from a for loop, which is incrementing to printing out each test as a separate table)
Any help would be appreciated, thanks

Answer 1

Change the select list for whatever columns you want to display, but this will limit the results as you want, for a given testid (replace testXYZ with the actual test you're searching on)
SELECT t.Test_name, s.*, sc.*
FROM Tests t
JOIN Scores sc
ON t.id_Tests = sc.Tests_id_Tests
JOIN Students s
ON sc.Students_id_Students = s.id_Students
WHERE t.id_Tests = 'textXYZ'
and sc.result <
(select avg(x.result)
from scores x
where sc.Tests_id_Tests = x.Tests_id_Tests)
Note: To run this for ALL tests, and have scores limited to those that are below the average for each test, you would just leave that one line out of the where clause and run:
SELECT t.Test_name, s.*, sc.*
FROM Tests t
JOIN Scores sc
ON t.id_Tests = sc.Tests_id_Tests
JOIN Students s
ON sc.Students_id_Students = s.id_Students
WHERE sc.result <
(select avg(x.result)
from scores x
where sc.Tests_id_Tests = x.Tests_id_Tests)

Answer 2

For example in PostgreSQL you could use a window function like AVG(Score) OVER (GROUP BY id_Tests), but in MySQL I suggest using a subquery as follows:
SELECT Scores.*, Students.*, t.Test_name, Avgscore
FROM Scores
JOIN Students ON sc.Students_id_Students = s.id_Students
JOIN
SELECT id_Tests, t.Test_name, AVG(sc.Result) AS Avgscore
FROM Tests t
JOIN Scores sc ON t.id_Tests = sc.Tests_id_Tests
-- WHERE id_Tests = $c
GROUP BY id_Tests, t.Test_name
) avgsc ON Scores.Tests_id_Tests=avgsc.id_Tests
WHERE Scores.Result < Avgscore
Note that a student can be listed multiple times if they got below average score multiple times -- might or might not be what you want.
I commented out the line filtering the test as I guess it is easier to retrieve all tests at once, but if you insist on filtering on one test on application level then you can filter here by uncommenting it.