DDL – Data Definition Language数据定义语言
DML – Data Manipulation Language数据操作语言
DCL – Data Control Language数据控制语言
TCL – Transaction Control Language
sql命令分类：
DDL:create,drop,alter
DML:select,insert,upodate,delete
DCL:grant,revoke
TCL:COMMIT,SAVEPOINT,ROLLBACK,SET TRANSACTION