原文：SQL 2005新增的几个函数之学习

今天学习了sql server 2005新增的几个函数,分别是row_number(),rank,DENSE_RANK,ntile,下面以例子
分别简单讲解之
1.row_number
   先来点数据,先建个表

SET NOCOUNT ON
CREATE TABLE Person(
FirstName VARCHAR(10),
Age INT,
Gender CHAR(1))
INSERT INTO Person VALUES ('Ted',23,'M')
INSERT INTO Person VALUES ('John',40,'M')
INSERT INTO Person VALUES ('George',6,'M')
INSERT INTO Person VALUES ('Mary',11,'F')
INSERT INTO Person VALUES ('Sam',17,'M')
INSERT INTO Person VALUES ('Doris',6,'F')
INSERT INTO Person VALUES ('Frank',38,'M')
INSERT INTO Person VALUES ('Larry',5,'M')
INSERT INTO Person VALUES ('Sue',29,'F')
INSERT INTO Person VALUES ('Sherry',11,'F')
INSERT INTO Person VALUES ('Marty',23,'F')
直接用例子说明问题

SELECT ROW_NUMBER() OVER (ORDER BY Age) AS [Row Number by Age],
FirstName,
Age
FROM Person
出现的数据如下

Row Number by Age    FirstName  Age
-------------------- ---------- -----------
1                    Larry      5
2                    Doris      6
3                    George     6
4                    Mary       11
5                    Sherry     11
6                    Sam        17
7                    Ted        23
8                    Marty      23
9                    Sue        29
10                   Frank      38
11                   John       40
 
可以观察到,是根据年龄升序排列了，并且row_number()是给出了序列号了,这个序列号被重命名为Row Number by Age,
 
如果不想按年龄排序,可以这样写

SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 1)) AS [Row Number by Record Set],
FirstName,
Age
FROM Person

另外一个例子

SELECT ROW_NUMBER() OVER (PARTITION BY Gender ORDER BY Age) AS [Partition by Gender], 
FirstName,
Age,
Gender
FROM Person

这里是按性别划分区间了，同一性别再按年龄来排序,输出结果如下
Partition by Gender  FirstName  Age         Gender
-------------------- ---------- ----------- ------
1                    Doris      6           F
2                    Mary       11          F
3                    Sherry     11          F
4                    Sue        29          F
1                    Larry      5           M
2                    George     6           M
3                    Sam        17          M
4                    Ted        23          M
5                    Marty      23          M
6                    Frank      38          M
7                    John       40          M

注意,姓名M开始,序号又从1,2,3开始了

2 RANK函数

先看例子

SELECT RANK() OVER (ORDER BY Age) AS [Rank by Age],
FirstName,
Age
FROM Person

输出如下

Rank by Age          FirstName  Age
-------------------- ---------- -----------
1                    Larry      5
2                    Doris      6
2                    George     6
4                    Mary       11
4                    Sherry     11
6                    Sam        17
7                    Ted        23
7                    Marty      23
9                    Sue        29
10                   Frank      38
11                   John       40

看到了么,同年岭的话,将有相同的顺序,顺序成1,2,2,4了

SELECT RANK() OVER (PARTITION BY Gender ORDER BY Age) AS [Partition by Gender], 

FirstName, Age, Gender FROM Person输出为

Partition by Gender  FirstName  Age         Gender
-------------------- ---------- ----------- ------
1                    Doris      6           F
2                    Mary       11          F
2                    Sherry     11          F
4                    Sue        29          F
1                    Larry      5           M
2                    George     6           M
3                    Sam        17          M
4                    Ted        23          M
4                    Marty      23          M
6                    Frank      38          M
7                    John       40          M

可以看到,按性别分组了,每个性别分组里,继续是用了rank函数
3 DENSE_RANK 函数
   SELECT DENSE_RANK() OVER (ORDER BY Age) AS [Dense Rank by Age],
       FirstName,
       Age
  FROM Person
输出结果为
Dense Rank by Age    FirstName  Age
-------------------- ---------- -----------
1                    Larry      5
2                    Doris      6
2                    George     6
3                    Mary       11
3                    Sherry     11
4                    Sam        17
5                    Ted        23
5                    Marty      23
6                    Sue        29
7                    Frank      38
8                    John       40
看到了么,和rank函数区别是,顺序始终是连续的,Doris 和George同年,都是排第2位,但之后的mary不象rank函数那样排第4,而是排第3位了
4 ntile函数

SELECT FirstName,
Age,
NTILE(3) OVER (ORDER BY Age) AS [Age Groups]
FROM Person

 输出

FirstName  Age         Age Groups
---------- ----------- --------------------
Larry      5           1
Doris      6           1
George     6           1
Mary       11          1
Sherry     11          2
Sam        17          2
Ted        23          2
Marty      23          2
Sue        29          3
Frank      38          3
John       40          3

这个函数按照ntile(n)中的N,把记录强制分成多少段,11条记录现在分成3段了,lary到mary是第1
段,sherry到maty是第2段,sue到john是第3段了

5   top n

  declare @n int

set @n=10;

select top(@n) * from table1