-- 附加费的价格和货币分开。金额仍在 amount，货币和计费单位在 price_unit，如 EUR/ITEM。可重复执行。
SET NAMES utf8mb4;

SET @ddl = (
  SELECT IF(
    EXISTS (
      SELECT 1 FROM information_schema.COLUMNS
      WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = 'pay_surcharge' AND COLUMN_NAME = 'price_unit'
    ),
    'SELECT 1',
    'ALTER TABLE pay_surcharge ADD COLUMN price_unit varchar(40) DEFAULT NULL COMMENT ''货币和计费单位，如EUR/ITEM、RMB/KG、%'' AFTER currency'
  )
);
PREPARE stmt FROM @ddl;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;