Splitting and distributing data from two tables into a new one - php

Question

I have been studying and learning PHP and MySQL and I have started a system that I'm developing for a friend's little school and to help me to improve my learning. I basically have in this case a table with the names of the students (tb_std) and another with the names of the teachers (tb_tch). The work is to distribute these students among the teachers in a new table, which is the way I think it will work better (tb_final).
I basically need each student to have a randomly chosen teacher so that the distribution is numerically even among the teachers.
In this example, I have 7 teachers and 44 students. Using SELECT query I did the operations to find out how many students would be for each teacher (add/division/mod), but how to make this draw to play in this new table I have no idea where to start.
1) Students (tb_std)
---------
id |std
1 | A1
2 | A2
3 | A3
(...)
44 | A44
2) Teachers (tb_tch)
--------
id |tch
--------
1 | T1
2 | T2
3 | T3
(...)
7 | T7
3) Final Combo (tb_final)
--------------------
id| student | teacher
--------------------
1 | A1 | T1
2 | A2 | T2
3 | A3 | T4
4 | A4 | T6
5 | A5 | T2
(...)
This is what I her in code until now:
<?php
session_start();
include_once("../conn/conexao.php");
$str_std="SELECT COUNT(*) as tstd FROM tb_std ";
$qry_std=mysqli_query($conn,$str_std);
while($res1=mysqli_fetch_assoc($qry_std)){
$v1 = $res1['tstd']; // 44 students
echo "Students: ".$res1['tstd']."<br>";
}
$str_tch="SELECT COUNT(*) as ttch FROM tb_tch ";
$qry_tch=mysqli_query($conn,$str_tch);
while($res2=mysqli_fetch_assoc($qry_tch)){
$v2 = $res2['ttch']; // 7 teachers
echo "Teachers ".$res2['ttch']."<br><br>";
}
$div = number_format($v1 / $v2);
$res = $v1 % $v2;
echo "Stud/Teach: ".$div."<br>"; // 6 std per teach
echo "Mod: ".$res."<br>"; // mod 2
?>
I have found some discussions including instructions CREATE and UNION, but I couldn't find other discussions around here that would help me with this division and distribution between tables to generate this new result.

Answer 1

You can solve this by next (a bit a complicate) query using window functions:
select
s.id, std student, tch teacher
from (
-- get Students with row_numbers
select
id,
row_number() over w as rn,
std
from tb_std
window w as (order by id)
) s
cross join (
-- get Teachers count and join to each student row
select count(*) tcnt from tb_tch
) tcnt
join (
-- select Teachers and join them to students depends row_number
-- divided to teachers count
select
row_number() over w as trn,
tch
from tb_tch
window w as (order by id)
) t on mod(s.rn, tcnt) = t.trn -1;
MySQL 8.0 fiddle
Result:
+====+=========+=========+
| id | student | teacher |
+====+=========+=========+
| 1 | S0 | T2 |
+----+---------+---------+
| 2 | S1 | T3 |
+----+---------+---------+
.........................
+----+---------+---------+
| 20 | S19 | T7 |
+----+---------+---------+
| 21 | S20 | T1 |
+----+---------+---------+
| 22 | S21 | T2 |
+----+---------+---------+