Get request doesnt work - php

Question

I have a form that queries my server for data using MySQL. I am using a form that sends get requests. It doesn't show anything and I don't know why. I is so strange because my query is valid and I tested it on PHPmyadmin..I am not striving for answers only, I want to know why this happened and what is the reason behind it.
Here is my code:
<form name="get" action="Roster.php" method="get">
<select name="course" id="course">
<?php
$get = mysqli_query($con, "SELECT teaching.Course_ID FROM `teaching` WHERE teaching.F_ID=213000000 ");
while ($row = mysqli_fetch_assoc($get)) {
echo '<option value ="' . $row["Course_ID"] . '"> ' . $row["Course_ID"] . ' </option>';
}
?>
</select>
<select name="group">
<?php
$get = mysqli_query($con, "SELECT `Group_ID` FROM `teaching` WHERE `teaching`.F_ID= 213000000");
while ($row = mysqli_fetch_array($get)) {
echo '<option value ="' . $row["Group_ID"] . '"> ' . $row["Group_ID"] . ' </option>';
}
?>
</select>
<date-util format="yyyy-mm-dd">
<label for="Date" > Date </label><input id="meeting" name="date" type="date" />
</date-util>
<input type="submit" name="Send" value="Get"/>
</form>
<?php
if ($_GET['submit']) {
$sql = " SELECT enrollment.S_ID,student.ID,student.F_Name,student.L_name,attendance.Status,attendance.Date
From enrollment
INNER JOIN student On enrollment.S_ID
INNER JOIN attendance On enrollment.S_ID
where enrollment.Course_ID =" . $_GET["course"] . "and enrollment.Group_ID =" . $_GET["group"] . "and attendance.date =" . $_GET["date"] . " ";
$result = mysqli_query($con, $sql);
$message = "Please Choose Course_ID and Group_ID ";
if ($result > 0) {
while ($row = mysqli_fetch_assoc($result)) {
echo "Hello";
echo "<tr>";
echo '<td>' . $row['ID'] . '</td>';
echo '<td>' . $row['F_Name'] . " " . $row['L.name'] . '</td>';
echo '<td>' . $row['Date'] . '</td>';
echo '<td>' . $row['Status'] . '</td>';
echo "</tr>";
}
} else {
echo "<tr>";
echo '<td>' . $message . '</td>';
}
}
?>

Answer 1

$_GET['submit'] does not exist. You need to add submit as your name attribute to your button like so
<input type="submit" name="submit" value="Get"/>
Also you should use prepared statements to prevent SQL injection attacks.