[Oracle]高效的SQL语句之分析函数(三)--row_number() /rank()/dense_rank()

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