Why is my gap using both insert and update? - php

Question

I want to update and insert using an if-else statement, but the query always goes "Insert" data. No "update" anyway. My code is below:
<form method="post" action="">
<label>ID: </label> <input type="text" name="id">
<label>Subject: </label> <input type="text" name="subject">
<input type="submit" name="submit">
</form>
<?php
$conn = new mysqli("localhost", "root", "", "zidm");
$id=$_POST['id'];
$subject=$_POST['subject'];
if (isset($_POST['submit'])){
$sql = "UPDATE exam SET $subject = '$marks' WHERE id = '$id'";
mysqli_query($conn,$sql);
echo "Data Updated";
}
else {
$sql="INSERT INTO exam ($subject, x_sess, x_class, x_exam, x_roll) VALUES ('$marks', '$ex_sess', '$class', '$exam', '$roll')";
mysqli_query($conn,$sql);
echo "Data Inserted";
}
?>

Answer 1

<form method="post" action="">
<label>ID: </label> <input type="text" name="id">
<label>Subject: </label> <input type="text" name="subject">
<input type="submit" name="submit">
</form>
<?php
$conn = new mysqli("localhost", "root", "", "zidm");
$id=$_POST['id'];
$subject=$_POST['subject'];
if (isset($_POST['submit'])){
$sql = "select id from exam WHERE id = $id";
$result = mysqli_query($conn,$sql);
if($result)
{
$sql = "UPDATE exam SET $subject = $marks WHERE id = $id";
mysqli_query($conn,$sql);
echo "Data Updated";
}
else
{
$sql="INSERT INTO exam ($subject, x_sess, x_class, x_exam, x_roll) VALUES ($marks, $ex_sess, $class, $exam, $roll)";
mysqli_query($conn,$sql);
echo "Data Inserted";
}
}
?>