如何使用“ Case When”和“ Group By”创建Mysql查询

Question

给出数据库中的3个表（prodi，mahasiswa和status），我试图输出分组和计数的结果。

Database creation SQL

查询

SELECT
prodi.namaprodi,
(case when count(`status`.idsm)='1' then 1 else 0 end) as '1',
(case when count(`status`.idsm)='2' then 1 else 0 end) as '2',
(case when count(`status`.idsm)='3' then 1 else 0 end) as '3',
(case when count(`status`.idsm)='4' then 1 else 0 end) as '4',
(case when count(`status`.idsm)='5' then 1 else 0 end) as '5',
(case when count(`status`.idsm)='6' then 1 else 0 end) as '6',
(case when count(`status`.idsm)='7' then 1 else 0 end) as '7',
(case when count(`status`.idsm)='8' then 1 else 0 end) as '8'
FROM
`status`
INNER JOIN mahasiswa ON mahasiswa.idm = `status`.idm
INNER JOIN prodi ON prodi.idp = mahasiswa.idp
GROUP BY
prodi.idp, `status`.idm

实际结果

Hukum   0   1   0   0   0   0   0   0
Hukum   0   0   0   0   0   1   0   0
Hukum   0   1   0   0   0   0   0   0
Ekonomi 0   0   0   1   0   0   0   0
Ekonomi 0   0   0   1   0   0   0   0

所需结果

Hukum   0   2   0   0   0   1   0   0
Ekonomi 0   0   0   2   0   0   0   0

有什么想法可以使该查询正常工作吗？

Answer 1

在分组之前将其放入子查询中

Maxprice

http://www.sqlfiddle.com/#!9/bf9dc9/10

Answer 2

查询必须写为

SELECT
    prodi.namaprodi, 
    SUM(Sem1) as Sem1,
    SUM(Sem2) as Sem2,
    SUM(Sem3) as Sem3,
    SUM(Sem4) as Sem4,
    SUM(Sem5) as Sem5,
    SUM(Sem6) as Sem6,
    SUM(Sem7) as Sem7,
    SUM(Sem8) as Sem8
FROM mahasiswa
INNER JOIN 
(SELECT status.idm, 
    (case when count(`status`.idsm)='1' then 1 else 0 end) as 'Sem1', 
    (case when count(`status`.idsm)='2' then 1 else 0 end) as 'Sem2', 
    (case when count(`status`.idsm)='3' then 1 else 0 end) as 'Sem3', 
    (case when count(`status`.idsm)='4' then 1 else 0 end) as 'Sem4', 
    (case when count(`status`.idsm)='5' then 1 else 0 end) as 'Sem5', 
    (case when count(`status`.idsm)='6' then 1 else 0 end) as 'Sem6', 
    (case when count(`status`.idsm)='7' then 1 else 0 end) as 'Sem7', 
    (case when count(`status`.idsm)='8' then 1 else 0 end) as 'Sem8' 
    FROM `status` 
    INNER JOIN mahasiswa ON mahasiswa.idm = `status`.idm 
    GROUP BY `status`.idm 
)SemCount
    ON mahasiswa.idm = SemCount.idm
INNER JOIN prodi ON prodi.idp = mahasiswa.idp
GROUP BY prodi.idp

我检查了查询，它给出了想要的结果。

当您需要按多个列进行分组时，在子查询中创建一个组，在主查询中创建另一个