Question

我在下面有mysql查询。但它不起作用。

SELECT  CASE
        WHEN isLevel = '1' THEN 'Mahasiswa'
        WHEN isLevel = '2' THEN 'Dosen'
        WHEN isLevel = '3' THEN 'Karyawan'
        ELSE 'Admin'
        END as level 
FROM mhs LEFT JOIN if(isLevel=1,'mahasiswa','dosen') ON username=iduser 
WHERE blabala.

如果“isLevel = 1”我想加入mhs和mahasiswa表，如果“isLevel = 2”加入mhs和dosen表，...我应该怎么写查询？

请帮帮我。感谢。

Answer 1

您无法有条件地加入。但是使用2个左连接来获取两个表

Select  CASE
    WHEN isLevel = '1' THEN 'Mahasiswa'
    WHEN isLevel = '2' THEN 'Dosen'
    WHEN isLevel = '3' THEN 'Karyawan'
ELSE 'Admin'
END as level 
from mhs 
left join mahasiswa m on m.username=iduser
left join dosen d on d.username=iduser

Answer 2

您可以在此处使用union语句：

SELECT 'Mahasiswa' AS level
FROM mhs, mahasiswa

WHERE isLevel = 1
AND username = iduser

UNION ALL

SELECT CASE WHEN isLevel = 2 THEN 'Dosen' 
            WHEN isLevel = 3 THEN 'Karyawan' ELSE 'Admin' END AS level
FROM mhs, dosen

WHERE isLevel != 1
AND username = iduser;

虽然这似乎很难做到。从长远来看，将isLevel列的翻译放入另一个表中可能会更好，因此可以重用。除非你真的很喜欢写CASE语句。

mysql查询：如何用语句连接多个表

2 个答案: