Question

刚学数据库，纯新手，用的oracle，有道题不会呀！求解！！！

Suppose you are given a relation grade points(grade, points), which provides
a conversion from letter grades in the takes relation to numeric scores; for
example an “A” grade could be specified to correspond to 4 points, an “A−”
to 3.7 points, a “B+” to 3.3 points, a “B” to 3 points, and so on. The grade
points earned by a student for a course offering (section) is defined as the
number of credits for the course multiplied by the numeric points for the
grade that the student received.
Given the above relation, and our university schema, write each of the
following queries in SQL. You can assume for simplicity that no takes tuple
has the null value for grade.
a. Find the total grade-points earned by the student with ID 12345,
across all courses taken by the student.
b. Find the grade-point average (GPA) for the above student, that is,
the total grade-points divided by the total credits for the associated
courses.
c. Find the ID and the grade-point average of every student.
求详细讲解呀！！！不要只给答案

Answer 1

you do not give the university schema which includes the relations of courses and courses selected by students. I don't know the number of credits and the student ID in which relation.

Suppose there exists two relations:
Students-course(SID, CID, grade)
course(CID, credit)
a: use a join operation
select sum(GP.points*C.credit) as GP
from Students-course SC, grade-points GP, course C
where SC.SID='12345' and SC.grade=GP.grade and SC.CID=C.CID
b: It need two aggragation operations
select sum(GP.points*C.credit)/sum(C.credit) as GPA
from Students-course SC, grade-points GP, course C
where SC.SID='12345' and SC.grade=GP.grade and SC.CID=C.CID
c: It need a group operaton and two aggragation operations
select SID, sum(GP.points*C.credit)/sum(C.credit) as GPA
from Students-course SC, grade-points GP, course C
where SC.grade=GP.grade and SC.CID=C.CID
group by SID