总和然后按SQL数据库中的查询分组

Question

Recently I am facing problem with the SUM() function and GROUP BY clause of SQL. I want to find the GPA (Grade Point Average) on the basis of float values.

The following is the code that I have been using so far:

SELECT semesterid,
       SUM( value ) AS TotalValue
FROM tbcourse
GROUP BY semesterid;

Answer 1

If value is the individual grade points, then the following will give you the GPA (Grade Point Average) for each value of semesterid .

SELECT semesterid,
       AVG( value ) AS TotalValue
FROM tbcourse
GROUP BY semesterid;

If you have any questions or comments, then please feel free to post a Comment accordingly.

Answer 2

You're on the right track using a GROUP BY with the SUM() aggregate function. Part of the trick here, though, is that you need to grab data (products of hours*points) for each class, which you get from an aggregate. So you need two "query steps", one where you aggregate the INPUT data for the division and then the step for calculating "points / credit hours".

My suggestion for fixing this sort of issue is to use an INNER QUERY. Basically, you use FROM(...) to write a second "inner" query to make a specific or custom set of data to SELECT from.

So what does that mean for your problem?

I wasn't sure what your data actually looks like, so I took a stab and assumed two tables. Since you have a semester ID and courses (which also have an ID I assume), I took that to mean you had at least those. I assumed some simple table structures and here's what I would do in your case following my advice:

Semesters Table

CREATE TABLE semesters (
  id INT(12) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  term_name VARCHAR(256) NOT NULL,
  term_year INT(8) NOT NULL
 );

Courses Table

CREATE TABLE courses (
  id INT(12) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(256) NOT NULL,
  semester_id INT(12) NOT NULL,
  credit_hours INT(4) NOT NULL,
  grade FLOAT,
  FOREIGN KEY (semester_id) REFERENCES semesters(id)
 );

Query 1 :: GROUP BY each semester to see GPA by semester

SELECT
  sg.semester_id AS "Semester"
  ,sg.grade_points AS GPs
  ,sg.hours AS hours
  ,ROUND((sg.grade_points / sg.hours), 2) AS GPA
FROM (
  SELECT
    c.semester_id AS semester_id
    ,SUM(c.credit_hours + 0.0) AS hours
    ,SUM((c.grade * c.credit_hours) + 0.0) AS grade_points
  FROM courses AS c
  GROUP BY c.semester_id
) as sg  # sg ~ SemesterGrades;

Query 2 :: # Query 2 - SUM over ALL semesters to get (weighted) cumulative GPA

SELECT
  ROUND((grades.grade_points / grades.hours), 2) AS GPA
FROM (
  SELECT
    c.semester_id AS semester_id
    ,SUM(c.credit_hours + 0.0) AS hours
    ,SUM((c.grade * c.credit_hours) + 0.0) AS grade_points
  FROM courses AS c
) as grades

I also made up an SQL-Fiddle so you can play around with this and, if you want, change the table structure and try out more queries to get it right. Also, feel free to ask for more help if I didn't answer your question! Hope it helps.

http://sqlfiddle.com/#!9/2b0693

UPDATE I don't believe the use of AVG() is correct for this problem. The reason is that GPA calculations are not a "standard" average - it's a weighted average. Said in other words, the AVG() is a special case of a weighted average where the weight on all the elements (credits) is the same (it's not for classes with different numbers of credit hours.