PHP import help

Hi,

I am very new to PHP and MySQL however I do not think this request is too difficult.

I am trying to build a php script that will allow my market managers to enter their company profit factors for the year. The form asks what country they work in and what their profit factors are. It then imports this data into a MYSQL database.

Some sample data looks like this

Market Profit Factor
UK              50
BE               200
DE              80

This works fine, however I would like to know how I can tell my script to just overwrite existing records. As If I enter a profit factor twice then I get two rows of data i.e -

Market Profit Factor
UK              50
UK              50

Is there anyway I can just overwrite the data?

I have attached my php

<?php

$host="localhost"; // Host name 
$username="root"; // Mysql username 
$password=""; // Mysql password 
$db_name="bonuscalc"; // Database name 
$tbl_name="market_values"; // Table name

// Connect to server and select database.
mysql_connect("$host", "$username", "$password")or die("cannot connect"); 
mysql_select_db("$db_name")or die("cannot select DB");

// Get values from form 
$market=$_POST['market'];
$profit_factor=$_POST['profit_factor'];


// Insert data into mysql 
$sql="INSERT INTO $tbl_name(market, profit_factor)VALUES('$market', '$profit_factor')";
$result=mysql_query($sql);

// if successfully insert data into database, displays message "Successful". 
if($result){
echo "Successful";
echo "<BR>";
echo "<a href='insert.php'>Back to main page</a>";
}

else {
echo "ERROR";
}

// close connection 
mysql_close();
?>

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