hive--TopN 案例
题目要求：
现在有这样一份数据：
1,huangxiaoming,45,a-c-d-f
2,huangzitao,36,b-c-d-e
3,huanglei,41,c-d-e
4,liushishi,22,a-d-e
5,liudehua,39,e-f-d
6,liuyifei,35,a-d-e
字段的意义：
id,name,age,favors
id,姓名,年龄,爱好
其中需要注意的是：每一条记录中的爱好有多个值，以"-"分隔
需求：
求出每种爱好中，年龄最大的两个人（爱好，年龄，姓名）
解题步骤：
1） 通过 explode() 函数将一条数据转为多行数据。
select id,name,age,h from hobby lateral view explode(split(hobby,"-")) hob as h; +-----+----------------+------+----+
| id | name | age | h |
+-----+----------------+------+----+
| 1 | huangxiaoming | 45 | a |
| 1 | huangxiaoming | 45 | c |
| 1 | huangxiaoming | 45 | d |
| 1 | huangxiaoming | 45 | f |
| 2 | huangzitao | 36 | b |
| 2 | huangzitao | 36 | c |
| 2 | huangzitao | 36 | d |
| 2 | huangzitao | 36 | e |
| 3 | huanglei | 41 | c |
| 3 | huanglei | 41 | d |
| 3 | huanglei | 41 | e |
| 4 | liushishi | 22 | a |
| 4 | liushishi | 22 | d |
| 4 | liushishi | 22 | e |
| 5 | liudehua | 39 | e |
| 5 | liudehua | 39 | f |
| 5 | liudehua | 39 | d |
| 6 | liuyifei | 35 | a |
| 6 | liuyifei | 35 | d |
| 6 | liuyifei | 35 | e |
+-----+----------------+------+----+2) 用开窗函数 row_number( ) over( )，按 h【爱好】分区，按age 排序，为每一行添加行号。
为了方便查看此处创建中间表hobby_bak：
create table hobby_bak as
select tt.id id,tt.name name,tt.age age,tt.h h,
row_number()over(partition by h order by age desc) as rownum
from
(select id,name,age,h from hobby
lateral view explode(split(hobby,"-")) hob as h
) as tt);结果如下：
+---------------+-----------------+----------------+--------------+-------------------+
| hobby_bak.id | hobby_bak.name | hobby_bak.age | hobby_bak.h | hobby_bak.rownum |
+---------------+-----------------+----------------+--------------+-------------------+
| 1 | huangxiaoming | 45 | a | 1 |
| 6 | liuyifei | 35 | a | 2 |
| 4 | liushishi | 22 | a | 3 |
| 2 | huangzitao | 36 | b | 1 |
| 1 | huangxiaoming | 45 | c | 1 |
| 3 | huanglei | 41 | c | 2 |
| 2 | huangzitao | 36 | c | 3 |
| 1 | huangxiaoming | 45 | d | 1 |
| 3 | huanglei | 41 | d | 2 |
| 5 | liudehua | 39 | d | 3 |
| 2 | huangzitao | 36 | d | 4 |
| 6 | liuyifei | 35 | d | 5 |
| 4 | liushishi | 22 | d | 6 |
| 3 | huanglei | 41 | e | 1 |
| 5 | liudehua | 39 | e | 2 |
| 2 | huangzitao | 36 | e | 3 |
| 6 | liuyifei | 35 | e | 4 |
| 4 | liushishi | 22 | e | 5 |
| 1 | huangxiaoming | 45 | f | 1 |
| 5 | liudehua | 39 | f | 2 |
+---------------+-----------------+----------------+--------------+-------------------+
3) 取行号小于3的记录。
select * from hobby_bak where rownum<3;+---------------+-----------------+----------------+--------------+-------------------+
| hobby_bak.id | hobby_bak.name | hobby_bak.age | hobby_bak.h | hobby_bak.rownum |
+---------------+-----------------+----------------+--------------+-------------------+
| 1 | huangxiaoming | 45 | a | 1 |
| 6 | liuyifei | 35 | a | 2 |
| 2 | huangzitao | 36 | b | 1 |
| 1 | huangxiaoming | 45 | c | 1 |
| 3 | huanglei | 41 | c | 2 |
| 1 | huangxiaoming | 45 | d | 1 |
| 3 | huanglei | 41 | d | 2 |
| 3 | huanglei | 41 | e | 1 |
| 5 | liudehua | 39 | e | 2 |
| 1 | huangxiaoming | 45 | f | 1 |
| 5 | liudehua | 39 | f | 2 |
+---------------+-----------------+----------------+--------------+-------------------+