检查数据库中的用户名并更新[html，mysql，php]

i have been trying since yesterday, and almost covered all questions regarding this matter in Stackoverflow plus googling, but so far nothing is working with me, i try to check username availability before updating the username in database, however, it wont check and always update the username directly without error message regarding not availability of the name..

here my code

//new connection
$con = new mysqli("localhost", "student", "student", "C14D5");
if ($con->connect_errno) { //failed
    echo "Failed to connect to MySQL: (" . $con->connect_errno . ") " . $con->connect_error;
}
//success 
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    if (isset($_POST['clientN'])) {
        $query = mysqli_query("SELECT client_name FROM clients WHERE client_name='".$_POST['clientN']."'");
        if (mysqli_num_rows($query) != 0) {
            echo "<script>
alert('Username is not available, please select another username.');
</script>";
            header('Location: '. $_SERVER['HTTP_REFERER']  );
        } else {
            // run sql 
            $sql ="UPDATE `clients` SET `client_name` = '".$_POST['clientN']."' WHERE `client_ID` = '".$_POST['SelectClient']."'";
            if ($con->query($sql) === TRUE) {
                echo "<h3> New record created successfully</h3>";
                header('Location: '. $_SERVER['HTTP_REFERER']  );
            } else {
                echo "Error : " . $sql  . "<br>" . $con->error;
            }
            $con->close();
        }
    }

You can use the mysqli_num_rows() function to avoid data duplication in your database

use this code :

//specify the database connection factors as usual ,then
$uname = $_POST['your_username_field'];    

$sql = "SELECT * FROM your_db where username='$uname'";
//the variable 'sql' will store the resultset of the query

$num_row = mysqli_num_rows($sql);
// the 'num_row' will store the number of rows which matches your $sql resultset. So if it is greater than '0' then the data already exists

if( $num_row > 0)
{
    // display 'username exists error'
}
else
{
   // Insert user name into your database table
}

If the num_rows is greater than 0 ,then the username is already present in your database table . So at that case throw error. else INSERT the user name into your database and display success message .