学步园

       row_number()over(partition by col1 order by col2)表示根据col1分组，在分组内部根据col2排序，而此函数计算的值就表示每组内部排序后的顺序编号（组内连续的唯一的）。
      与rownum的区别在于：使用rownum进行排序的时候是先对结果集加入rownum然后再进行排序，而此函数在包含排序从句后是先排序再计算行号码。
row_number()和rownum差不多，功能更强一点（可以在各个分组内从1开始排序）。   

create table employee (empid int ,deptid int ,salary decimal(10,2));
insert into employee values(1,10,5500.00);
insert into employee values(2,10,4500.00);
insert into employee values(3,20,1900.00);
insert into employee values(4,20,4800.00);
insert into employee values(5,40,6500.00);
insert into employee values(6,40,14500.00);
insert into employee values(7,40,44500.00);
insert into employee values(8,40,55500.00);
insert into employee values(9,50,6500.00);
insert into employee values(10,50,7500.00);
commit;

SQL> SELECT T.*,ROW_NUMBER() OVER(PARTITION BY deptid ORDER BY deptid) FO
  2  from employee t;

     EMPID     DEPTID     SALARY         FO
----------         ----------       ----------       ----------
         1         10                    5500          1
         2         10                    4500          2
         3         20                    1900          1
         4         20                    4800          2
         5         40                    6500          1
         8         40                    55500        2
         6         40                    14500        3
         7         40                    44500        4
         9         50                    6500          1
        10         50                   7500          2