device_id(field)
123asf15fas
456g4fd45ww
7861fassd45

...
$q = "INSERT INTO $database.$table `device_id` VALUES $device_id";
$result = mysql_query($q);
...

<?php
    // YOUR MYSQL DATABASE CONNECTION
    $hostname = 'localhost';
    $username = 'root';
    $password = '';

    $database = 'device';
    $table = 'device_id';
    $db_link = mysql_connect($hostname, $username, $password);
    mysql_select_db( $database ) or die('ConnectToMySQL: Could not select database: ' . $database );
    //$result = ini_set ( 'mysql.connect_timeout' , '60' );

    $device_id = $_GET["device_id"];
    $q = "REPLACE INTO $database.$table (`device_id`) VALUES ($device_id)";
    $result = mysql_query($q);
    if (!$result) {
        die('Invalid query: ' . mysql_error());
    }
?>

SELECT * FROM `your_table` WHERE `device_id`='123asf15fas'

$result = mysql_query("SELECT * FROM `your_table` WHERE `device_id`='123asf15fas'");
if (!$result)
{
   // your code INSERT
   $result = mysql_query("INSERT INTO $database.$table `device_id` VALUES $device_id");
}

ALTER TABLE `your_table` ADD UNIQUE (`device_id`)

#1062 - Duplicate entry '1' for key 'PRIMARY'

REPLACE INTO $database.$table SET device_id = $device_id;