ALTER TABLE `invoices`
  ADD COLUMN `document_type` VARCHAR(30) NOT NULL DEFAULT 'invoice' AFTER `number`,
  ADD COLUMN `original_invoice_id` BIGINT UNSIGNED NULL AFTER `offer_id`,
  ADD INDEX `invoices_original_invoice_id_index` (`original_invoice_id`),
  ADD CONSTRAINT `invoices_original_invoice_id_foreign` FOREIGN KEY (`original_invoice_id`) REFERENCES `invoices` (`id`) ON DELETE SET NULL;