PHP MySQL Selecting Multiple Tables Based on Column/Row Value

I am just doing some testing with a few tables that I'm trying to relate together, based on the values of my 'students' table.

Here are the total tables that I have:

students TABLE

id, name, course_num, msg_num

biology TABLE

id, details, msg_num

chemistry TABLE

id, details, msg_num

physics TABLE

id, details, msg_num

So I'm trying to select the 'students' table first, and based on the 'course_num' & 'msg_num' values in the 'students' table, it will determine which table is selected out of an array of tables that I have (as shown above, the 'biology', 'chemistry', & 'physics' tables).

The 'students' table looks like this:

The 'biology' table has data like this:

The 'chemistry' table looks like this:

The 'physics' table looks like this:

So, once again, I'm trying to have it to where if the number in 'course_num' in the 'students' table is set to either '0,1,2' , then it will dynamically select the related table from the array list that I have set up in advance. Here is the exact code that I've tried below, but it is NOT syncing up properly. Please let me know what I'm doing wrong here:

<div style="position:relative; margin:auto; width:400px; font-family:Poppins; font-size:16px; border:2px solid black; padding:10px; margin-top:100px;">


<?php

include 'db_connect.php';


$selected_courses = array("biology", "chemistry", "physics");



$sql1 = "SELECT * FROM students";
$result1 = $conn->query($sql1);

while($row1 = $result1->fetch_assoc()){
    $course = $selected_courses[$row1['course_num']];

    $sql2 = "SELECT students.*, $course.*, FROM students, $course
    WHERE students.msg_num = $course.msg_num";

    $result2 = $conn->query($sql2);
    $row2 = $result2->fetch_assoc();

    echo "STUDENT NAME: ". $row2['name'] . "<br>";
    echo "DETAILS: ". $row2['details'] . "<br>";
    echo "MSG NUM: ". $row2['msg_num'] . "<br>";
    echo "<br>";
}


$conn->close();

?>

</div>