DDL：数据定义语言 --- CREATE、ALTER、DROP、TRUNCATE

TPL：事务处理语言 --- COMMIT、ROLLBACK、SAVEPOINT、SET TRANSACTION

DCL：数据控制语言 --- GRANT、REVOKE

DML：数据操作语言 --- SELECT、UPDATE、INSERT、DELETE

join从句：： inner | full outer | left outer | right outer | cross

内连接inner join: select a.user_name,a.over,b.over from user1 a join user2 b on a.user_name=b.user_name;