Define UNIQUE key constraint for the table - php

Question

I have implemented this code to insert and update the data in my database but when I try it a new row's data is always being inserted in the table and it is not being updated How can I define the unique key in my case to get it to work? I have the primary key id.
<?php
$json = '{"latitude":53.86898451,"longitude":10.66561387,"route":4}';
$data = json_decode ( $json );
$route = "route_" . $data->{'route'};
$latitude = $data->{'latitude'};
$longitude = $data->{'longitude'};
require 'connection.php';
// check whether route's table exist.
$results = $con->query ( "SHOW TABLES LIKE'" . $route . "'" ) or die ( mysqli_error () );
if (($results->num_rows) == 1) {
//"UPDATE MyGuests SET lastname='Doe' WHERE id=2"
$sql = "INSERT INTO ".$route."(latitude, longitude, created_at)
VALUES( ? , ? , NOW() )
ON DUPLICATE KEY
UPDATE latitude = VALUES(latitude)
, longitude = VALUES(longitude)";
$stmt = $con->prepare($sql) or die ( $con->error );
$stmt->bind_param("ss",$latitude,$longitude);
$stmt->execute();
$stmt->close();
echo "Table exist";
} else {
$create = "CREATE TABLE " . $route . "
(id INT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY UNIQUE,
latitude FLOAT(10,6) NOT NULL,
longitude FLOAT(10,6) NOT NULL,
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP)" ;
$stmt = $con->prepare($create) or die ( $con->error );
$stmt->execute();
$stmt->close();
echo "table was craeted";
}
UPDATE
This is what I'm trying now
$sql = "INSERT INTO ".$route."(id, latitude, longitude, created_at)
VALUES( id, ? , ? , NOW() )
ON DUPLICATE KEY
UPDATE latitude = VALUES(latitude)
, longitude = VALUES(longitude)";
Which variable should I add in the VALUES braces for id?

Answer 1

You are not specifying a value for id in your INSERT INTO statement, so there will never be a duplicate key.
Instead, it is auto-generating the next available ID.
To solve this, specify the value of the pre-existing ID (if any).