1 薪水排序后薪水排名在第3-5的员工
1）select * from(select ename,sal,rownum rn from
(select ename,sal from emp_44 where sal is not null order by sal desc)where rownum<6)where rn>2;
2) select * from (select ename,sal,rownum rn from (select enam,sal from emp_44 where sal is not null order by sal desc)) where rn between 3 and 5;
2 删除一张表中所有数据的方式
1 truncate table 命令将快速删除数据表中的所有记录
2 delete 产生 rollback ，如果删除在数据量的表速度会很慢，同时会占用很多的rollback，segments.truncate 是DDL操作，不产生rollback，速度会快些。
3 用一条sql语句取出所有姓名有重复的学员姓名和重复的记录数
Select name,count(*) from student group by name having count(*)>1 order by count(*)desc;
4 去除oracle数据库 表中重复数据应有如下两种方法：
方法一：创建新表删除旧表法
1）create student1 as select distinct id,name,score from student;
2）drop table student;
3）rename strudent1 to student;
方法二：使用rowid（地址）伪列
删除伪列地址除了最大地址值以外的记录
delete from temp where rowid not in(select max(rowid) from temp group by id);
删除伪列地址除了最小地址值以外的记录
delete from temp where rowid not (select min(rowid) from temp group by id);