Taking mean of all students for one subject using php - php

Question

I have an SQL table of students with one subject. I would like to take the mean of all the students for this particular subject such that:
Sum of the subject/Number of total students
The table looks like this:
Student ID : 1
=============
Maths : 40
Student ID : 2
=============
Maths : 60
Student ID : 3
=============
Maths : 90
Student ID : 4
=============
Maths : 0
So if the student has scored 0, then ignore this student and its score in calculating the mean.
<?php
if(!isset($_SESSION["loggedin"]) || $_SESSION["loggedin"] !== true){
header("location: login.php");
exit;
}
$db = new PDO("mysql:host=localhost; dbname=db;charset=utf8",'user','');
$sql = "SELECT * FROM Students";
$stmt = $db->query($sql);
while($data = $stmt->fetch(PDO::FETCH_OBJ)){
//How to take the mean of 1 subject for all students?
}
?>

Answer 1

Fix this line first so that...you can only get scores greater that 0
$sql = "SELECT * FROM students WHERE subjectscore > 0"; //assuming your column for scores is "subjectscore"
The the rest of the code to get mean should be
$stmt = $db->query($sql);
while($data = $stmt->fetch(PDO::FETCH_OBJ)){
$sumparts .= $data->subjectscore."+"; //Add a delimiter after every returned obj
}
print_r($sumparts); //For debug :to see which ones are selected
$sumarr = explode("+",$sumparts); // Convert the delimited string to array
$sum = array_sum($sumarr); //Get sum of values of array in this case the scores
$divisor = count($sumarr) - 1; //The minus 1 is necessary since well the delimited string will always have 1 extra key therefore making the count to count 1 more unnnecessary key
$total = $sum/$divisor; //Sum of all divided by the total number of objects
echo $total;