MYSQL逗号分隔的结果有两种类型

Question

我有四张桌子，如下所示。

表：类

ClassID     |   ClassSTD
--------------------------------
1           |   STD-1
2           |   STD-2
3           |   STD-3
4           |   STD-4

表：科

SectionId   |   SectionName | ClassId
--------------------------------------------
1           |   sec-A       | 1
2           |   sec-B       | 1
3           |   sec-C       | 1
4           |   sec-A       | 2
5           |   sec-B       | 2
6           |   sec-C       | 2
7           |   sec-A       | 3

表：Subject

subjectId   |   subjectName
------------------------------------
1           |   Art
2           |   Music
3           |   Play

表SubjectAllocationToClass

classId     |   sectionID           |   subjectId   | type
-----------------------------------------------------------------------
1(STD-1)        |   1(sec-A)            |   1(Art)      | main
1(STD-1)        |   2(sec-B)            |   1(Art)      | main
1(STD-1)        |   3(sec-C)            |   1(Art)      | optional
1(STD-1)        |   1(sec-A)            |   2(Music)    | main
1(STD-1)        |   2(sec-B)            |   2(Music)    | optional

上表"SubjectAllocationToClass“显示了类的两种主题类型(主主题和可选主题)的分布情况。

需要所有类而不分区段，subjectAllocation需要所有区段而不需要subjectAllocation

如何从SELECT语句中获得以下结果？

    classSTD |  sectionName | Main subjectName   | Optional subjectName
    -----------------------------------------------------------------------------
    STD-1    |  sec-A       | Art, Music         |
    STD-1    |  sec-B       | Art                |  Music
    STD-1    |  sec-C       |                    |  Art
    STD-3    |  sec-A       |                    |
    STD-4    |              |                    |

Answer 1

select 
  ClassSTD as ClassSTD, 
  sectionname AS SectionName, 
  COALESCE(GROUP_CONCAT(CASE WHEN sac.type = 'main' THEN subjectName END), '')  as 'Main subjectname',
  COALESCE(GROUP_CONCAT(CASE WHEN sac.type = 'optional' THEN subjectName END), '')  as 'Optional subjectname'
FROM SubjectAllocationToClass sac
JOIN  Class c  ON c.classid = sac.classid
JOIN Section sc ON sc.sectionid = sac.sectionid
JOIN Subject sj ON  sj.subjectid = sac.subjectid
GROUP BY ClassSTD, SectionName;

Answer 2

可以使用GROUP_CONCAT()和CASE表达式进行条件分组，如下所示

select c.ClassID, s.sectionName,
xx.`Main subjectName`,
xx.`optional subjectName`
from Class c
join Section ss on c.ClassID  = ss.ClassID     
join (
select sa.sectionID,
group_concat(case when sa.type = 'main' then s.subjectName else null end) as  `Main subjectName`,
group_concat(case when sa.type = 'optional' then s.subjectName else null end) as  `optional subjectName`
from SubjectAllocationToClass sa
join Subject s on sa.subjectId = s.subjectId   
group by sa.subjectId ) xx on ss.SectionId = xx.SectionId;