SQL概念：嵌套循环

Question

1. Teacher (userID, name, DOB, gender, email, address, phoneNo, password, highestQual, isAdmin)
2. Lesson (lessonID, lessonFee, <teacherID>, <subjectID>)
3. Enroll (<studentID>, <lessonID>, <paymentID>,result)

假设运行以下SQL将返回两个具有相同teacherID的元组。

SELECT teacherID FROM tuition.Lesson WHERE lessonID IN(
                SELECT lessonID FROM tuition.Lesson WHERE lessonID NOT IN ((
                SELECT distinct lessonID FROM tuition.Enroll ))
                ); 



但是如果我想选择要显示的名称，结果将返回一个元组而不是两个元组。

SELECT name FROM tuition.teacher WHERE userID IN (
                SELECT teacherID FROM tuition.Lesson WHERE lessonID IN(
                SELECT lessonID FROM tuition.Lesson WHERE lessonID NOT IN ((
                SELECR distinct lessonID FROM tuition.Enroll )))
                );  

我可以做什么，以便我的结果显示两个元组，同一个名称显示两次。请帮忙指教。

Answer 1

tuition.teacher只有userID teacher1的记录，所以你得到一条记录。这完全正确。您应该在第一个查询中使用distinct子句，以获得teacherID

之类的唯一SELECT distinct teacherID FROM tuition.Lesson ...

如果您强烈要复制结果，请使用{/ 1}}子句，如

UNION