计算查询中的GPA

Question

I'm having some trouble connecting the tables on this one to get a single result. Here's how you calculate GPA:

1. Get the average grade for each course.
2. Multiply each average by the number of credit hours in said course.
3. Add the results from 2.
4. Divide the result from 3 by the total number of credit hours.

I have three relevant tables:

CREATE TABLE Student
(
SSN        INT(11),
SName    VARCHAR(25),
Major    VARCHAR(25),
PRIMARY KEY (SSN)
);

CREATE TABLE Course
(
CNO        INT(11),
CName    VARCHAR(3),
CreditHour    TINYINT(4),
PRIMARY KEY (CNO)
);

CREATE TABLE Grade
(
SSN        INT(11),
CNO        INT(11),
Grade        TINYINT(4)
);

Unfortunately, SQL fiddle seems to be down, so I can't make a fiddle for this. How would I make a select statement to list the student info with a calculated gpa column? Every time I attempt this, I end up with an overblown set of nested subqueries, like this:

SELECT student.*, (total/hours) as gpa WHERE hours = 
   (SELECT SUM(course.creditHours) AS hours 
   FROM course, student, grade 
   WHERE course.cno = grade.cno 
   AND grade.ssn = student.ssn 
   GROUP BY course.cno) 
   AND total = (SELECT SUM(AVG(grade.grade)*course.creditHours) as total 
   FROM grade, course, student 
   WHERE grade.ssn = student.ssn 
   AND grade.cno = course.cno)

That doesn't actually appear to be going in the right direction, either. What should I be doing to derive this column?

Answer 1

If you want the GPA by Student, I would calculate as follows:

1 Multiply the number of credit hours for each course by the Grade and add up the product, per Student

2 Divide the result from 1 by the total number of credit hours, per student.

SELECT
    s.SSN,
    SUM(c.CreditHour * g.Grade) / SUM(c.CreditHour) Average_GPA
FROM Student s
INNER JOIN Grade g on s.SSN = g.SSN
INNER JOIN Course c on g.CNO = c.CNO
GROUP BY s.SSN;

If you simply want the weighted average GPA for each course, then the following query can be used using the steps from your question:

1.Get the average grade for each course.

2.Multiply each average by the number of credit hours in said course.

3.Add the results from 2.

4.Divide the result from 3 by the total number of credit hours.

SELECT
    c.CNO,
    SUM(c.CreditHour * average_gpa.GPA) / SUM(c.CreditHour) Weighted_Average_GPA
FROM Course c
INNER JOIN
(
SELECT
    CNO,
    AVG(g.Grade) GPA
FROM Grade
GROUP BY CNO
) average_gpa
ON c.CNO = average_gpa.CNO
GROUP BY c.CNO;