Computing an average from different tables - php

Question

I am sorry for a verlo long question, just trying to explain in details. My formatting is not very good, sorry for that as well. I had a PHP/ MySQL App that essentially was not truly relational as I had one large table for all student scores. Among other things, I was able to calculate the average score for each subject, such that the average appeared alongside a student's score. Now I have since split the table up, to have a number of tables which I am successfully querying and creating School Report Cards as before. The hardship is that I can no longer calculate the avaerages for any subject.
Since I had one table with 5 subjects and each of the subjects had 2 tests, I queried for data and calculated the average as follows:
The one table (Columns):
id date name exam_no term term year eng_mid eng_end mat_mid mat_end phy_mid phy_end bio_mid bio_end che_mid che_end
The one query:
$query = "SELECT * FROM pupils_records2
WHERE grade='$grade' && class='$class' && year = '$year' && term ='$term'";
$result = mysqli_query($dbc, $query);
if (mysqli_num_rows($result) > 0) {
$num_rows=mysqli_num_rows($result);
while($row = mysqli_fetch_array($result)){
//English
$eng_pupils1{$row['fname']} = $row['eng_mid'];
$eng_pupils2{$row['fname']} = $row['eng_end'];
$mid=(array_values($eng_pupils1));
$end=(array_values($eng_pupils2));
$add = function($a, $b) { return $a + $b;};
$eng_total = array_map($add, $mid, $end);
foreach ($eng_total as $key => $value){
if ($value==''){
unset ($eng_total[$key]);
}
}
$eng_no=count($eng_total);
$eng_ave=array_sum($eng_total)/$eng_no;
$eng_ave=round($eng_ave,1);
//Mathematics
$mat_pupils1{$row['fname']} = $row['mat_mid'];
$mat_pupils2{$row['fname']} = $row['mat_end'];
$mid=(array_values($mat_pupils1));
$end=(array_values($mat_pupils2));
$add = function($a, $b) { return $a + $b;};
$mat_total = array_map($add, $mid, $end);
foreach ($mat_total as $key => $value){
if ($value==''){
unset ($mat_total[$key]);
}
}
print_r($mat_total);
$mat_no=count($mat_total);
echo '<br />';
print_r($mat_no);
$mat_ave=array_sum($mat_total)/$mat_no;
$mat_ave=round($mat_ave,1);
}
}
//Biology
etc
I split the table into separate tables and have names in a separate table, not needed for calculating avaerages, so I will not show it here. Each subject table tajkes the following form:
id date exam_no term year grade class test*
*Test would be eng_mid or eng_end or mat_mid etc.
Because I had only one query which returned 10 rows (5 subjects each with two tests: e.g. eng_mid (English Mit exam), eng_end (english end of term test), I was able to capture all rows in one call and pack each subject into an array, and then work out the class average, with the help of array_map. It may not be elegant, but it worked very well. Now, I have each test in it's own table.
I was trying to write a joint so as to get a signle resultset but the query fails. The columns as like:
I know that the database design is not anything to be proud off, but coming from a huge single table, this is a massive step (worthy a pat on the shoulder).
What I wish to do is to be able to query all my data and calculate class averages (about 30 students in each class). I tried to use separate queries but I ran into a wall, in that previously I would use the WHILE conditional as shown after the query for it to pull all rows and create an array from which I could get desired results. Now several queries just makes me confused as to how I can archieve the same results since a join is not working. Also I am having a separate $row variable, and that throws me further off balance!
Is it even possible to do averages as I did on my infamous one table (from the dark side) or is my table design so messed up, what I want just isn't humanly possible?
Please any help will be deeply appreciated.

Answer 1

Try using union. It would be something like
select grade, test from math
union all
select grade, test from english
union all
....
Also, in my opinion, better design would be to have table exams something like that (warning, pseudo-DML):
id int primary key,
student_id int foreign key students
subject_id int foreign key subjects
exam_type_id int foreign key exam_types
grade int(????)
exam_types table would be just midterm and final, but you'll be able to easily support more types in future, if required.
subjects table will store all kinds of subjects you have (at this time there will be only five of them: math, eng, phy, etc.
The averaging query would be as simple as (yes, you can actually do aggregation in the query itself)
select student_id, avg(grade)
from exams
group by student_id