oracle的执行计划
SQL> EXPLAIN PLAN FOR SELECT * FROM emp;
已解释。
SQL> SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE'));
或者：
SQL>  select * from table(dbms_xplan.display);


select distinct * from student;   --排除重复对象 
select * from(select e.*,rownum rw from emp e) where rw>=8 and rw<=10;
--分页查询
select * from (select e.*,dense_rank() over(order by sal desc) rank from emp e) where rank=5;   ---查出指定条件的一个排名


select job , max(sal) from emp having max(sal)>2000 group by job
          -----------------------数据库group by和having


select months_between(sysdate,hiredate) from emp; 查看入职日期时间
select last_day(sysdate) from dual;      查看本月的最后一天


select substr(ename,0,1) from emp where regexp_like(substr(ename,0,1),'^([C-M]+)$');     -------截取首字母为C到M的值


update students set s=trim(ssss); ---把数据复制到新的字段上去


update students set sold=to_date('2013/06/30', 'yyyy-mm-dd hh24:mi') where sid =1;
     ------------------------修改表数据


alter table students add (ssss number(20));   ------添加表字段


alter table students rename column ssss to ss;   ------修改表字段


alter table students drop column sss;        ----------删除表字段


show recyclebin;            ------查看回收站        purge recyclebin;-------------清空回收站
show recyclebin;       -------闪回查看              flashback table 表名 to before drop; ---闪回数据库


savepoint a; 定义保存点      rollback to savepoint a;  ---回退回保存点


set transaction isolation level SERIALIZABLE(read only(只读)、read committed(读取提交数据)); ---设置事物的隔离级别


创建两张关联表create table AA 
(
 aid number(10) primary key ,
 sname varchar2(40) not null,
 hibernate date
);
create table BB( 
 bid number(10) primary key,
 dname varchar2(50),
  constraint id_fk foreign key(bid) references AA(aid)
);


或者：创建两张表，然后修改表结构 alter table BB add contraint id_fk foreign key(bid) references AA(aid); 
alter table grade add constraint id_fk  foreign key (id) references student (id)