MySQL与GROUP CONCAT和FIND_IN_SET一起使用

Question

我有3表的MySQL数据库，如下所示

表： subject_master

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

表： class_Master

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

表： subjectAllocation

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

如何获取每个主题明智的className列表

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

我学习了GROUP_CONCAT和FIND_IN_SET，但是不能一起使用，如下所示

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

您应该在ON子句中使用FIND_IN_SET来匹配列表中而不是SELECT子句中的元素。

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