sql - 关系代数

Question

我想知道我是否可以获得一些关于我所做的关系代数的反馈，以及任何改进它的建议，如果您发现我的 SQL 有任何问题，您能否指出来。

SELECT CourseName
FROM Course
WHERE CourseNo = 6;

∏类(class)名称(σCourseNo=6(类(class)))

SELECT CU.UnitNo
FROM C.Course, CU CourseUnit
WHERE C.CourseName = CU.CourseNo
AND C.CourseName = 'Research';

∏UnitNo(CourseUnit(σC​​ourseName='Research'(类(class))))

SELECT U.UnitName
FROM S Student, SU StudentUnit, U Unit
WHERE S.StudentNo = SU.StudentNo
AND SU.UnitNo = U.UnitNo
AND S.StudentName = 'John Perry';

∏UnitName(Unit(StudentUnit(σStudentName='John Perry'(学生))))

SELECT count(S.StudentNo) ResearchStudents
FROM C Course, S Student
WHERE C.CourseNo = S.CourseNo
AND C.CourseName = 'Research';

∏ResearchStudents((C)count(StudentNo)(学生(σCourseName='Research'(类(class)))))

Answer 1

from 子句有问题，我删除了 C.Course 之间的一个点

SELECT CU.UnitNo
FROM C Course, CU CourseUnit
WHERE C.CourseName = CU.CourseNo
AND C.CourseName = 'Research';