说明：  万常选版数据库系统原理与设计实验教程第1-4次实验的参考答案，全部自己写的，经过验证。包括简单查询、复杂查询、数据定义操作、数据更新操作。
(The experiment1-4 solution for Wan s the principle of database systems and design of experiments tutorial , all write their own proven, to be verified. involve simple queries, complex queries, data definition operations, data update operation.)