clean_db_uninstall.php

<?php
global $wpdb;
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::RATING_ITEM_ENTRY_TBL_NAME );
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::RATING_ITEM_ENTRY_VALUE_TBL_NAME );
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::RATING_ITEM_TBL_NAME );
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::RATING_FORM_TBL_NAME );
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::RATING_RESULT_TBL_NAME );

// custom fields
$custom_fields = MRP_Multi_Rating_API::get_custom_fields( null );
foreach ( $custom_fields as $custom_field ) {
    $wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . 'mrp_custom_field_' . $custom_field['custom_field_id'] );
}
$wpdb->query( 'DROP TABLE IF EXISTS ' . $wpdb->prefix . MRP_Multi_Rating::CUSTOM_FIELDS_TBL_NAME );