Reading data from two tables on a same database, using "if - else" - php

Question

ok guys I need a bit of help with this, remaking this to format ir properly.
I have this code:
(this is accessing a table called "students" in database.)
<?php
require_once('connection.php');
$id = $_SESSION['SESS_MEMBER_ID'];
$result3 = mysql_query("SET NAMES UTF8;");
$result3 = mysql_query("SELECT * FROM students where mem_id='$id'");
while($row3 = mysql_fetch_array($result3))
{
$fname = $row3['fname'];
$country = $row3['country'];
$class = $row3['class'];
$headteacher = $row3['headteacher'];
$attendance = $row3['attendance'];
$homework = $row3['homework'];
$messagestudent = $row3['messagestudent'];
}
?>
<?php
if($class=='K1')
echo "teacher 1";
else if($class=='K2')
echo "teacher 2";
else if($class=='K3')
echo "teacher 3";
?>
In another table, i have the teacher names and what i need to do and i cannot find the right way to do it, is to call the teacher's name from the table "teachers" after the query confirms the K1, K2 or K3 data on the "students" table column "class".
Basically what i need is to change the contents of the echo part, switching it from static data needed to be within the code, to a data contained on another table, for example. both tables have a column called "class", so if class column for a student says "K1" i want this to then go check the "teacher" table's column "class" and pick the one that matches "K1" and display it in the result echo, I'm sure it is possible, but not with my current skill level.
The table structure for students is:
mem_id, username, password, fname, country, class, attendance, homework, messagestudent
The table structure for teacher is:
mem_id, class, name, comment
Hope you guys shed some light on me!
Thanks in advance.
PS. I know the query is using the deprecated mysql_, but when I tried to change it to mysqli_ following a guide, it never worked.

Answer 1

If I understood, you can get the desired data through this query (without JOIN or if statement):
SELECT students.*,teachers.name FROM students,teachers WHERE students.id='$id' AND students.class = teachers.class
It returns the teacher name according the student class, at the same row.

Answer 2

You can use left join for this. Try using following query:
SELECT * FROM students as st
LEFT JOIN teachers AS ts
on st.class=ts.class
WHERE st.mem_id = '$id'
You don't need to use if-else statement for this. And it will also work with mysqli.
According to your script, it should work like:
<?php
require_once('connection.php');
$id = $_SESSION['SESS_MEMBER_ID'];
$result3 = mysql_query("SET NAMES UTF8;");
$result3 = mysql_query("SELECT st.*, ts.name AS teacher_name FROM students as st LEFT JOIN teachers AS ts on st.class=ts.class WHERE st.mem_id = ".$id);
while($row3 = mysql_fetch_array($result3))
{
$fname = $row3['fname'];
$country = $row3['country'];
$class = $row3['class'];
$headteacher = $row3['headteacher'];
$attendance = $row3['attendance'];
$homework = $row3['homework'];
$messagestudent = $row3['messagestudent'];
$teachername = $row3['teacher_name'];
}
echo $class. ' - '. $teachername;
?>