转载 http://www.cnblogs.com/icebutterfly/archive/2009/08/05/1539657.html
语法：ROW_NUMBER() OVER(PARTITION BY COLUMN ORDER BY COLUMN)
简单的说row_number()从1开始，为每一条分组记录返回一个数字，这里的ROW_NUMBER() OVER (ORDER BY xlh DESC) 是先把xlh列降序，再为降序以后的没条xlh记录返回一个序号。
示例：
xlh row_num
1700 1
1500 2
1085 3
710 4
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伪列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开时排序）．
rank()是跳跃排序，有两个第二名时接下来就是第四名（同样是在各个分组内）．
select rank() over(order by sale / cnt desc) as sort, sale / cnt
from (select -60 as sale, 3 as cnt
from dual
union
select 24 as sale, 6 as cnt
from dual
union
select 50 as sale, 5 as cnt
from dual
union
select 20 as sale, 2 as cnt
from dual
union
select 40 as sale, 8 as cnt from dual);
----
SORT SALE/CNT
1 10
1 10
3 5
4 4
5 -20
dense_rank()l是连续排序，有两个第二名时仍然跟着第三名。相比之下row_number是没有重复值的 ．
select dense_rank() over(order by sale / cnt desc) as sort, sale / cnt
from (select -60 as sale, 3 as cnt
from dual
union
select 24 as sale, 6 as cnt
from dual
union
select 50 as sale, 5 as cnt
from dual
union
select 20 as sale, 2 as cnt
from dual
union
select 40 as sale, 8 as cnt from dual);
----
SORT SALE/CNT
1 10
1 10
2 5
3 4
4 -20
lag（arg1,arg2,arg3):
实例：
初始化数据
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(8,50,6500.00)
insert into employee values(9,50,7500.00)
数据显示为
empid deptid salary
----------- ----------- ---------------------------------------
1 10 5500.00
2 10 4500.00
3 20 1900.00
4 20 4800.00
5 40 6500.00
6 40 14500.00
7 40 44500.00
8 50 6500.00
9 50 7500.00
需求：根据部门分组，显示每个部门的工资等级
预期结果：
empid deptid salary rank
----------- ----------- --------------------------------------- --------------------
1 10 5500.00 1
2 10 4500.00 2
4 20 4800.00 1
3 20 1900.00 2
7 40 44500.00 1
6 40 14500.00 2
5 40 6500.00 3
9 50 7500.00 1
8 50 6500.00 2
SQL脚本：
SELECT *, Row_Number() OVER (partition by deptid ORDER BY salary desc) rank FROM employee