sql - 关系代数

Question

我想知道我是否可以得到一些关于我所做的关系代数的反馈以及关于改进它的任何建议，如果你发现我的 SQL 有任何问题，你能不能指出来。

SELECT CourseName
FROM Course
WHERE CourseNo = 6;

∏类(class)名称(σCourseNo=6(Course))

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

∏UnitNo(CourseUnit(σC​​ourseName='Research'(Course)))

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'(Student))))

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

∏ResearchStudents((C)count(StudentNo)(Student (σCourseName='Research'(Course)))

Answer 1

from 子句中存在问题，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';