group by and group_concat alternative - php

Question

Using GROUP_CONCAT() usually invokes the group-by logic and creates temporary tables, which are usually a big negative for performance. Sometimes you can add the right index to avoid the temp table in a group-by query, but not in every case.
https://stackoverflow.com/a/26225148/9685125
After Reading this post, I realized that I was doing wrong, because many time I made complicated query using huge GROUP_CONCAT(). Such as
GROUP_CONCAT(DISTINCT exam.title) AS exam,
GROUP_CONCAT(subject.title, '<br/> Th - ', mark.th, ' | PR - ', mark.pr SEPARATOR ',') AS mark
But what can be alternative of GROUP_CONCAT in following situation without using subquery. I mean using only Mysql join,
For example, let see two relational database and and query to explain my problem
Student
id | Rid | name
========================
1 | 1 | john
Marks
id | std_id | th
======================
1 | 1 | 60
2 | 1 | 70
3 | 1 | 80
4 | 1 | 90
"SELECT
student.en_name, mark.th
FROM student
JOIN mark ON student.id = mark.std_id
WHERE student.id=:id;"
Column would be repeated if only use JOIN
John: 60, John: 70, John: 80, John: 90
So, I use GROUP BY. But if I assign GROUP BY to student.id, only first row is fetched
"SELECT
student.en_name, mark.th
FROM student
JOIN mark ON student.id = mark.std_id
WHERE student.id=:id
GROUP BY student.id;"
Result is
John: 60
So to get result, we have to assign that group.concat
"SELECT
student.en_name,
GROUP_CONCAT(mark.th) as mark
FROM student
JOIN mark ON student.id = mark.std_id
WHERE student.id=:id
GROUP BY student.id;"
And final and expected result using exploding array
$name=$row['en_name'];
echo "$name: <br/>";
$mrk_array = explode(',',$row['mark']);
foreach($mrk_array as $mark){
echo $mark.", ";
}
John:
60, 70, 80, 90,
Here, I don't see any alternative of GROUP_CONCAT to fetch all associated value of each Id and prevent duplicate, please help me how to replace GROUP_CONCAT from here.
Also, one friend told me
So why GROUP_CONCAT if you're "exploding" it. You might as well return a nice associative array and then deal with displaying it there.
But I can't understand, what he means ?

Answer 1

Too long for a comment...
With your original query, you are effectively returning an array of rows (associative arrays):
array(array('en_name' => 'John', 'mark' => 60),
array('en_name' => 'John', 'mark' => 70),
array('en_name' => 'John', 'mark' => 80),
array('en_name' => 'John', 'mark' => 90)
)
When you use GROUP BY and GROUP CONCAT, you are effectively imploding the 'mark' elements of that array to
array('en_name => 'John', 'mark' => '60,70,80,90')
and you then have to explode the array again to process the data.
If you stick with your original query, you can instead do the imploding in your application framework e.g.
$name = "";
while ($row = $result->fetch()) {
if ($row['en_name'] != $name) {
$name = $row['en_name'];
echo "$name: <br/>" . $row['mark'];
}
else {
echo ',' . $row['mark'];
}
}
Output:
John:
60,70,80,90
This will generally be a lot faster than using GROUP BY and GROUP_CONCAT in the database.