数据库汇总

复习

1. 数据库的密码加密：
   （1）单向加密：insert into users (username,password) values ('ms',MD5('123456'));
   （2）双向加密：insert into users (username,password) values ('ms',ENCODE('123','456'));

2. SQL聚合查询
   select user_id from user group by user_id;

3. SQL筛选查询select * from user where user_id in (select user_id from user group by user_id having count(user_id)>1)order by user_id desc;

4. SQL衔接查询：这里写代码片