This SQL statement will truncate a number of tables in Magento including Orders, Credit Memo’s and Tags, before executing this SQL statement please make sure you backup your database!

We have used this script on Magento 1.7.0.2

SET FOREIGN_KEY_CHECKS=0;
TRUNCATE `sales_flat_creditmemo`;
TRUNCATE `sales_flat_creditmemo_comment`;
TRUNCATE `sales_flat_creditmemo_grid`;
TRUNCATE `sales_flat_creditmemo_item`;
TRUNCATE `sales_flat_invoice`;
TRUNCATE `sales_flat_invoice_comment`;
TRUNCATE `sales_flat_invoice_grid`;
TRUNCATE `sales_flat_invoice_item`;
TRUNCATE `sales_flat_order`;
TRUNCATE `sales_flat_order_address`;
TRUNCATE `sales_flat_order_grid`;
TRUNCATE `sales_flat_order_item`;
TRUNCATE `sales_flat_order_payment`;
TRUNCATE `sales_flat_order_status_history`;
TRUNCATE `sales_flat_quote`;
TRUNCATE `sales_flat_quote_address`;
TRUNCATE `sales_flat_quote_address_item`;
TRUNCATE `sales_flat_quote_item`;
TRUNCATE `sales_flat_quote_item_option`;
TRUNCATE `sales_flat_quote_payment`;
TRUNCATE `sales_flat_quote_shipping_rate`;
TRUNCATE `sales_flat_shipment`;
TRUNCATE `sales_flat_shipment_comment`;
TRUNCATE `sales_flat_shipment_grid`;
TRUNCATE `sales_flat_shipment_item`;
TRUNCATE `sales_flat_shipment_track`;
TRUNCATE `sales_invoiced_aggregated`;
TRUNCATE `sales_invoiced_aggregated_order`;
TRUNCATE `sales_order_aggregated_created`;
TRUNCATE `sendfriend_log`;
TRUNCATE `tag`;
TRUNCATE `tag_relation`;
TRUNCATE `tag_summary`;
TRUNCATE `wishlist`;
TRUNCATE `log_quote`;
TRUNCATE `report_event`;
ALTER TABLE `sales_flat_creditmemo` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_creditmemo_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_invoice_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_address` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_payment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_order_status_history` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote` AUTO_INCREMENT=1; 
ALTER TABLE `sales_flat_quote_address` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_address_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_item_option` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_payment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_quote_shipping_rate` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_comment` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_grid` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_item` AUTO_INCREMENT=1;
ALTER TABLE `sales_flat_shipment_track` AUTO_INCREMENT=1;
ALTER TABLE `sales_invoiced_aggregated` AUTO_INCREMENT=1;
ALTER TABLE `sales_invoiced_aggregated_order` AUTO_INCREMENT=1;
ALTER TABLE `sales_order_aggregated_created` AUTO_INCREMENT=1;
ALTER TABLE `sendfriend_log` AUTO_INCREMENT=1;
ALTER TABLE `tag` AUTO_INCREMENT=1;
ALTER TABLE `tag_relation` AUTO_INCREMENT=1;
ALTER TABLE `tag_summary` AUTO_INCREMENT=1;
ALTER TABLE `wishlist` AUTO_INCREMENT=1;
ALTER TABLE `log_quote` AUTO_INCREMENT=1;
ALTER TABLE `report_event` AUTO_INCREMENT=1;
SET FOREIGN_KEY_CHECKS=1;