DENSE_RANK() 、RANK() 和 ROW_NUMBER() 运用

关键字: oracle：DENSE_RANK() 、RANK() 和 ROW_NUMBER()  --转载自javaeye sun

有些时候我们希望得到指定数据中的前n列，示例如下：

]得到每个部门薪水最高的三个雇员：

先创建示例表

create table emp  
as  
select * from scott.emp;  
  
alter table emp  
add constraint emp_pk  
primary key(empno);  
  
create table dept  
as  
select * from scott.dept;  
  
alter table dept  
add constraint dept_pk  
primary key(deptno);

先看一下row_number() /rank()/dense_rank()三个函数之间的区别

select emp.deptno,emp.sal,emp.empno,row_number() over (partition by deptno order by sal desc) row_number,  --1,2,3  
 rank() over (partition by deptno order by sal desc) rank, --1,1,3  
 dense_rank() over (partition by deptno order by sal desc) dense_rank from emp --1,1,2 

结果如下:

 
10    5000.00    7839    1    1    1  
10    2450.00    7782    2    2    2  
10    1300.00    7934    3    3    3  
20    3000.00    7788    1    1    1  
20    3000.00    7902    2    1    1  
20    2975.00    7566    3    3    2  
20    1100.00    7876    4    4    3  
20    800.00    7369    5    5    4  
30    2850.00    7698    1    1    1  
30    1600.00    7499    2    2    2 

取每个部门的薪水前三位雇员：

select t.deptno,t.rank,t.sal from  
 (  
 select emp.*,row_number() over (partition by deptno order by sal desc) row_number,  --1,2,3  
  rank() over (partition by deptno order by sal desc) rank, --1,1,3  
  dense_rank() over (partition by deptno order by sal desc) dense_rank from emp --1,1,2  
 ) t  
where t.rank<=3

 

结果如下：

 
10    1    5000.00  
10    2    2450.00  
10    3    1300.00  
20    1    3000.00  
20    1    3000.00  
20    3    2975.00  
30    1    2850.00  
30    2    1600.00  
30    3    1500.00

 

如果想输出成deptno  sal1   sal2   sal3这种类型的格式
步骤一（decode）：

select t.deptno,decode(row_number,1,sal) sal1,decode(row_number,2,sal) sal2,decode(row_number,3,sal) sal3 from  
 (  
 select emp.*,row_number() over (partition by deptno order by sal desc) row_number,  --1,2,3  
  rank() over (partition by deptno order by sal desc) rank, --1,1,3  
  dense_rank() over (partition by deptno order by sal desc) dense_rank from emp --1,1,2  
 ) t  
where t.rank<=3 

结果如下：

 
10    5000          
10                  2450      
10                             1300  
20    3000          
20                  3000      
20                              2975  
30    2850          
30                 1600      
30                             1500 

步骤二（使用聚合函数去除null,得到最终结果）：

select t.deptno,max(decode(row_number,1,sal)) sal1,max(decode(row_number,2,sal)) sal2,max(decode(row_number,3,sal)) sal3 from  
 (  
 select emp.*,row_number() over (partition by deptno order by sal desc) row_number,  --1,2,3  
  rank() over (partition by deptno order by sal desc) rank, --1,1,3  
  dense_rank() over (partition by deptno order by sal desc) dense_rank from emp --1,1,2  
 ) t  
where t.rank<=3  
group by t.deptno

  

结果如下：

10    5000    2450    1300  
20    3000    3000    2975  
30    2850    1600    1500