各位大佬！

Question

题目是用子查询查询成绩不及格的设计MySchool数据库课程的学生学号，姓名

select StudentNo,StudentName,GradeId

from Student

where GradeId=(

select GradeID

from Subject

where SubjectName='设计MySchool数据库' and SubjectID in(

select SubjectID

from Result

where StudentResult<60 

)

)

select StudentNo,StudentName,GradeID

from Student

where StudentNo in(

select StudentNo

from Result

where  StudentResult<60 and SubjectID =(

select SubjectID

from Subject

where SubjectName='设计MySchool数据库'

)

)

为什么条件倒换一下答案就错了，

下面的一段代码是对的

Answer 1

select StudentNofrom Resultwhere  StudentResult<60 在学生成绩表result里面每个学生的编号肯定是唯一的，这个是OK的而上面那段select SubjectIDfrom Resultwhere StudentResult<60 在学生成绩表里面你能保证学科id是唯一的吗

Answer 2

如果实在搞不清楚，建议GROUP BY看下或者将每个子查询摘出来单独查看下结果就会一目了然

Answer 3

SubjectID in(select SubjectIDfrom Resultwhere StudentResult<60 和StudentNo in(select StudentNofrom Result这两个能一样吗