I am using the following script to add a table to the wordpress database, but nothing is happening. I am not sure what I am doing wrong.

Any advice would be much appreciated.

     function vw_postcode_checker_create_db ()
    {
        global $wpdb;
      global $tableprefix;

        // Create Table Name
        $table_name =$tableprefix . "vw_postcode_checker";

        // Write Query to add table to database
        if ($wpdb->get_var('SHOW TABLES LIKE'.$table_name) !=$table_name)
        {
                $sql = "CREATE TABLE " . $table_name . " (
                    id INTEGER(10) UNSIGNED AUTO_INCREMENT,
                    postcode  varchar(250)   NOT NULL,
                    area varchar(250)   NOT NULL
                    PRIMARY KEY (id)
                )";
    // Execute Query to Create Table
    require_once (ABSPATH . 'wp-admin/includes/upgrade.php');
                dbDelta($sql);
                add_option ('vw_postcode_checker_version', '0.1');
        }
        }
// Register table function
register_activation_hook (__FILE__,'vw_postcode_checker_create_db');