MySql 的@符号定义一个变量在sql里的占位符作用

set @orderNo := 'order3';      先定义一个@开头的变量值为 order3

SELECT * t_order WHERE order_no = @orderNo;      查询订单号为 order3 的订单

DELETE FROM t_order WHERE order_no = @orderNo;    删除订单号为 order3 的订单

delete from t_order_item where order_id = (select id from t_order where order_no = @orderNo);