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:
- select count(a.id)
- 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
- 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：
- [email protected] | ["20150620","20150619"] |
- | [email protected] | ["20150816"] |
- | [email protected] | ["20150606","20150608","20150607","20150609","20150613","20150610","20150616","20150615"] |
- | [email protected] | ["20150827","20150623","20150627","20150820","20150823","20150612","20150717"] |
- | [email protected] | ["20150701","20150829","20150626","20150625","20150726","20150722","20150629","20150824","20150716","20150 |
- | [email protected] | ["20150804","20150803","20150801","20150809","20150807","20150806","20150905","20150904","20150730","20150 |
- | [email protected] |["20150803","20150801","20150809","20150808","20150805","20150806","20150906","20150904","20150730","20150 |
- | [email protected] | ["20150604","20150609","20150622","20150827","20150625","20150620","20150613","20150610","20150614","20150 |
- | [email protected] | ["20150902"] |
- | [email protected] | ["20150709"]
所以我们就可以按照这个返回的数组做文章，即为
- 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：
- Hive不允许直接访问非group by字段；
- 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；
- 使用数字下标，可以直接访问数组中的元素；
转载：http://blog.csdn.net/liyantianmin/article/details/48262109