ORDERS表在ORDER_ID列有一个主键约束。 ORDER_ITEMS表在ORDER_ID列上有一个外键约束，参考的是ORDERS表上的主键，这个约束定义的为DELETE CASCADE类型。 表ORDERS里存在ORDER_TOTAL列小于1000的行。 哪三个DELETE语句会执行成功。 A DELETE * FROM orders WHERE order_total<1000; B DELETE order_id FROM orders WHERE order_total<1000; C DELETE FROM orders WHERE order_total<1000; D DELETE orders WHERE order_total<1000; E DELETE FROM orders;