Question

我有两张桌子：

Catalog(ID, CourseName, PreReq, Professor)  
PreviousCourses(Username, PreviousCoursesID, Grade)

PreviousCoursesID对应于目录中的ID。我试图使用这种关系访问CourseName。

这是我的问题：

Select CourseName from Catalog where ID = (Select PrevCoursesID from PreviousCourses where Username = 'admin')

执行但返回空集。

Select PrevCoursesID from PreviousCourses where Username = 'admin'查询本身会返回PrevCoursesID，所以我不明白为什么我的嵌套查询无效。

Answer 1

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需要使用＆＃34; IN＆＃34;而不是＆＃34; =＆＃34;像这样

Select CourseName from Catalog where ID = (Select PrevCoursesID from PreviousCourses where Username = 'admin')

因为它是一组值而不是单个值。检查超过1个值时，需要使用IN关键字。这与

相同

Select CourseName from Catalog where ID IN (Select PrevCoursesID from PreviousCourses where Username = 'admin')

嵌套查询未按预期执行

1 个答案: