How to display a students rank out of hundred students using PHP & MySQL?

Question

I'm trying to get the rank of a certain student out of a class of a hundred, I want to be able to display the students rank.

I'm trying to get the rank of a student by grades and grade points. For example if student 1 has 2 A+ grades with a total of 10 points and student 2 has 3 B- grades with a total of 10 points student 1 will rank higher. I was wondering how would I be able to do this using PHP & MySQL?

Here is the PHP & MySQL code I have so far.

$gp = array();
$dbc = mysqli_query($mysqli,"SELECT grades.*, homework_grades.*, users_homework.*
                             FROM grades 
                             LEFT JOIN homework_grades ON grades.id = homework_grades.grade_id
                             LEFT JOIN users_homework ON homework_grades.users_homework_id = users_homework.id
                             WHERE users_homework.user_id = '$user_id'
                             AND users_homework.grade_average = 'A+'");

if (!$dbc) {
    print mysqli_error($mysqli);
} else {
    while($row = mysqli_fetch_array($dbc)){ 
    $gp[] = $row['grade_points'];
}
}

echo array_sum($gp);

Here is my MySQL tables.

CREATE TABLE homework_grades ( 
id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
grade_id INT UNSIGNED NOT NULL, 
users_homework_id INT UNSIGNED NOT NULL, 
user_id INT UNSIGNED NOT NULL, 
PRIMARY KEY (id) 
);



CREATE TABLE grades ( 
id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
letter_grade VARCHAR NOT NULL, 
grade_points FLOAT UNSIGNED NOT NULL DEFAULT 0, 
PRIMARY KEY (id) 
);



CREATE TABLE users_homework (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
user_id INT UNSIGNED NOT NULL,
homework_content LONGTEXT NOT NULL,
grade_average VARCHAR DEFAULT NULL,
PRIMARY KEY (id)
);

Answer 1

UPDATED

You need an aggregate. Do a SUM() and COUNT(). COUNT will tell you number of assignments, sum will tell you the total grade points. GROUP BY user_id to get this for each student.

$gp = array();
$dbc = mysqli_query($mysqli,"SELECT SUM(grade_points) as grade_points, user_id, count(*) as num_assignments
                             FROM grades 
                             LEFT JOIN homework_grades ON grades.id = homework_grades.grade_id
                             LEFT JOIN users_homework ON homework_grades.users_homework_id = users_homework.id
                             GROUP BY user_id
                             ORDER BY grade_points DESC , num_assignments ASC'");

if (!$dbc) {
    print mysqli_error($mysqli);
} else {
    $i=1;
    while($row = mysqli_fetch_array($dbc)){ 
        $user[$row['user_id']] = $i++;
        $rank[] = $row['user_id'];
        $gp[] = $row['grade_points'];
    }
}

// the rank of user 10 is
echo "the rank of user_id 10 is {$user[10]}";
echo "the rank of all users are: " ;
print_r($rank);

This will add up the grade points, and count the number of assignments. Sorting will make sure that a student with 10 points from 3 assignments, will have a higher rank than a student with 10 points from 4 assignments.

See the COUNT SUM and GROUP BY aggregates.