# 分析函数之row_number over(partition by col1 order by col2)

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）．

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。

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);

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

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

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

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;

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

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

DEPTNO         SAL RANK_ORDER
---------- ---------- ----------
10        1300           1
10        2450           2
10        5000           3
20         800           1
20        1100           2
20        2975           3
20        3000           4
20        3000           4
30         950           1
30        1250           2
30        1250           2
30        1500           4
30        1600           5
30        2850           6

select deptno,sal,dense_rank() over(partition by deptno order by sal) as dense_rank_order from scott.emp order by deptn;

DEPTNO         SAL DENSE_RANK_ORDER
---------- ---------- ----------------
10        1300                 1
10        2450                 2
10        5000                 3
20         800                 1
20        1100                 2
20        2975                 3
20        3000                 4
20        3000                 4
30         950                 1
30        1250                 2
30        1250                 2
30        1500                 3
30        1600                 4
30        2850                 5

select deptno,ename,sal,lag(ename,1,null) over(partition by deptno order by ename) as lag_ from scott.emp order by deptno;

DEPTNO ENAME                        SAL LAG_
---------- -------------------- ---------- --------------------
10 CLARK                       2450
10 KING                        5000 CLARK
10 MILLER                      1300 KING
20 JONES                       2975 FORD
20 SCOTT                       3000 JONES
20 SMITH                        800 SCOTT
30 ALLEN                       1600
30 BLAKE                       2850 ALLEN
30 JAMES                        950 BLAKE
30 MARTIN                      1250 JAMES
30 TURNER                      1500 MARTIN
30 WARD                        1250 TURNER

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