Select subjects from table - php

Question

I have a table called Grades and inside I have some columns:
Id id_student Subjects grade
3 1 [->] Biology 8
4 1 [->] Math 4
5 1 [->] Sports 4
6 1 [->] Math 8
7 1 [->] English 9
8 4 [->] Sports 10
9 4 [->] English 7
I selected everything from this table where id_student = student logged in
$sth = $this->dbh->prepare("SELECT * FROM note WHERE id_elev = :id_elev");
$sth->bindParam(":id_elev", $_SESSION['id']);
$sth->execute();
$result = $sth->fetch(PDO::FETCH_ASSOC);
So all I selected is this:
Id id_student Subjects grade
3 1 [->] Biology 8
4 1 [->] Math 4
5 1 [->] Sports 4
6 1 [->] Math 8
7 1 [->] English 9
I want to make a average grade for Math , Biology , etc for this student but I want it without making a SELECT * FROM Grades WHERE $result['Subjects'] == 'Biology' because some students may not have this subject in their schedule.
I want something like this:
Id id_student Subjects grade
4 1 [->] Math 4
6 1 [->] Math 8
But without WHERE $results['Subjects'] == 'Math';
I'm sorry it is hard to explain when you don't know a lot of english. I hope that someone understands what I wrote and give me an advice. I can make another table called Subject if it is necessary.

Answer 1

Check out this fiddle:
http://sqlfiddle.com/#!2/8ee5a/3
I think it does what you need. If not, it's a great place to test the query.
I used the GROUP BY function AVG() to get the average grade for each subject.
http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_avg
Here is the schema and query:
CREATE TABLE grades
(
Id int auto_increment primary key,
id_student int,
Subjects varchar(20),
grade int(1)
);
INSERT INTO grades
(Id, id_student, Subjects, grade)
VALUES
('3', '1', 'Biology', 8),
('4', '1', 'Math', 4),
('5', '1', 'Sports', 4),
('6', '1', 'Math', 8),
('7', '1', 'English', 9);
SELECT id_student, Subjects, AVG(grade) FROM grades GROUP BY Subjects;

Answer 2

The query below is an example of the use I mentioned in my comment
-- The AVG() function returns the average value of a numeric column.
-- because you are executing an aggregate function on the Grade column you have to specify which columns to group the results together by.
SELECT
Id
,Subject
,AVG(Grade) as 'Average Grade'
FROM
Result
GROUP BY
Id
,Subject