Hive 的collect_set使用详解

﹏ヽ暗。殇╰゛Y 2022-06-07 06:19 157阅读 0赞

转载：http://blog.csdn.net/liyantianmin/article/details/48262109

有这么一需求，在hive中求出一个数据表中在某天内首次登陆的人；可以借助collect\_set来处理sql:

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1.  select count(a.id)   
2.  from (select id,collect\_set(time) as t from t\_action\_login where time**<**='20150906' group by id) as a where size(a.t)=1 and a.t\[0\]='20150906';

上面中的

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1.  select id,collect\_set(time) as t from t\_action\_login where time**<**='20150906' group by id

会按照id分组，因为一个id可能对应一天也可能对应多天，对应多天表示有多天都有登陆，所以一个id会对应多个日期time,通过collect\_set会把每个id所对应的日期构建成一个以逗号分隔的数组返回。上述SQL返回：

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1.    123@163.com                                                                                      | \["20150620","20150619"\]                                                                                    |  
2.  | abc@163.com                                                                                      | \["20150816"\]                                                                                               |  
3.  | cde@qq.com                                                                                       | \["20150606","20150608","20150607","20150609","20150613","20150610","20150616","20150615"\]                  |  
4.  | 789@sohu.com                                                                                     | \["20150827","20150623","20150627","20150820","20150823","20150612","20150717"\]                             |  
5.  | 987@163.com                                                                                      | \["20150701","20150829","20150626","20150625","20150726","20150722","20150629","20150824","20150716","20150 |  
6.  | ddsf@163.com                                                                                     | \["20150804","20150803","20150801","20150809","20150807","20150806","20150905","20150904","20150730","20150 |  
7.  | 182@163.com                                                                                      |\["20150803","20150801","20150809","20150808","20150805","20150806","20150906","20150904","20150730","20150 |  
8.  | 22225@163.com                                                                                    | \["20150604","20150609","20150622","20150827","20150625","20150620","20150613","20150610","20150614","20150 |  
9.  | 18697@qq.com                                                                                     | \["20150902"\]                                                                                               |  
10. | 1905@qq.com                                                                                      | \["20150709"\]

所以我们就可以按照这个返回的数组做文章，即为

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1.  where size(a.t)=1 and a.t\[0\]='20150906';

表示某id所对应的数组长度为1 并且第一个时间为20150906的id表示为该天首次登陆。

总结：

1.  Hive不允许直接访问非group by字段；
2.  对于非group by字段，可以用Hive的collect\_set函数收集这些字段，返回一个数组；
3.  使用数字下标，可以直接访问数组中的元素；

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