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)

SQL> SELECT G.*, ROW_NUMBER() OVER(PARTITION BY a ,b ORDER BY c DESC) ROWN
2??? from (select '1' a, '2' b, '1' c
3??????????? from dual
4????????? union all
5????????? select '1', '2', '2'
6??????????? from dual
7????????? union all
8????????? select '1', '3', '3'
9??????????? from dual
10????????? union all
11????????? select '1', '3', '4'
12??????????? from dual
13????????? union all
14????????? select '1', '4', '5' from dual
15????????? union all
16????????? select '1','3','5'from dual) G
17 ;

A B C?????? ROWN
- - - ----------
1 2 2????????? 1
1 2 1????????? 2
1 3 5????????? 1
1 3 4????????? 2
1 3 3????????? 3
1 4 5????????? 1

6 rows selected

　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）．

　　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 ．

　　lag（arg1,arg2,arg3):
arg1是从其他行返回的表达式
arg2是希望检索的当前行分区的偏移量。是一个正的偏移量，时一个往回检索以前的行的数目。
arg3是在arg2表示的数目超出了分组的范围时返回的值。

语句一:

select row_number() 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);

执行结果:

????????? SORT?????? SALE/CNT
---------- ----------
???????????? 1???????????? 10
???????????? 2????????????? 5
???????????? 3????????????? 4
???????????? 4??????????? -10
???????????? 5??????????? -20

?

语句二:查询员工的工资,按部门排序

select ename,sal,row_number() over (partition by deptno order by sal desc) as sal_order from scott.emp;

执行结果:

ENAME?????????????????????????? SAL????? SAL_ORDER
-------------------- ---------- ----------
KING?????????????????????????? 5000????????????? 1
CLARK????????????????????????? 2450????????????? 2
MILLER???????????????????????? 1300????????????? 3
SCOTT????????????????????????? 3000????????????? 1
FORD?????????????????????????? 3000????????????? 2
JONES????????????????????????? 2975????????????? 3
ADAMS????????????????????????? 1100????????????? 4
SMITH?????????????????????????? 800????????????? 5
BLAKE????????????????????????? 2850????????????? 1
ALLEN????????????????????????? 1600????????????? 2
TURNER???????????????????????? 1500????????????? 3
WARD?????????????????????????? 1250????????????? 4
MARTIN???????????????????????? 1250????????????? 5
JAMES?????????????????????????? 950????????????? 6

已选择14行。

语句三:查询每个部门的最高工资

select deptno,ename,sal from
???? (select deptno,ename,sal,row_number() over (partition by deptno order by sal desc) as sal_order
???????? from scott.emp) where sal_order <2;

执行结果:

?????? DEPTNO ENAME????????????????????????? SAL
---------- -------------------- ----------
?????????? 10 KING????????????????????????? 5000
?????????? 20 SCOTT???????????????????????? 3000
?????????? 30 BLAKE???????????????????????? 2850

已选择3行。

语句四：

select deptno,sal,rank() over (partition by deptno order by sal) as rank_order from scott.emp order by deptno;

执行结果:

???? D