MySQL use of GROUP CONCAT and FIND_IN_SET together

Question

I have MySQL DB with 3 tables as below

Table: subject_master

-----------------------
subjectId   |   subjectName
--------------------------------------
1           |   English
2           |   German
3           |   French
4           |   Spanish

Table: class_Master

----------------------------------
classId     | className
---------------------------------------
1           |   Alpha
2           |   Beta
3           |   Gama
4           |   laser

Table: subjectAllocation

-------------------------------------------------------
allocationId    |   classId         | subjectId     
------------------------------------------------------
1               |   1(Alpha)        |   1,2,3
2               |   2 (Beta)        |   2,3
3               |   4 (laser)       |   4,2,1

how to get list of className per subject wise

subjectName     |   classNames
-------------------------------------
English         |   Alpha, laser
German          |   Alpha, Beta, laser
French          |   Beta,Gama
Spanish         |   laser

I learn, GROUP_CONCAT & FIND_IN_SET but not able to use then together as below

SELECT
a.subjectName, FIND_IN_SET(c.className , (GROUP_CONCAT(b.classId))
FROM
subject_master a
LEFT JOIN subjectAllocation b ON a.subjectId = b.sectionId
LEFT JOIN class_master  c ON b.classId = c.classId
GROUP BY a.subjectId

Answer 1

You should use FIND_IN_SET in the ON clause to match an element in a list, not in the SELECT clause.

SELECT a.subjectName, GROUP_CONCAT(c.className) AS classNames
FROM subject_master AS a
LEFT JOIN subjectAllocation AS b ON FIND_IN_SET(a.subjectId, b.subjectId)
LEFT JOIN class_Master AS c ON b.classId = c.classId
GROUP BY a.subjectId