how do i check if the value of a column is null, and only then execute the query? for example:

col1 col2 col3
01   abc  

i run a query which first checks if the record exists or not; if it exists, it should execute the update query and if it doesn't exist, it executes the insert query. how do i check if col3 is null and if it is null, it should execute the update query. .

$sql = "SELECT uid FROM `users` WHERE uid = '" . $user_id . "'";
    $result = mysql_query($sql,$conn) or die('Error:' .mysql_error());

    $totalrows = mysql_num_rows($result);
    if($totalrows < 1)
    {
        insertUser($user_id,$sk, $conn);

    }
    else
    {
        updateSessionKey($user_id,$sk,$conn);
    }