内容来源：http://blog.csdn.net/azhao_dn/article/details/6921429
在hive上执行查询：
select count(*) from user_active_vv_20110801_31 where active_type_3>0
UNION ALL
select count(*) from user_active_vv_20110801_31 where active_type_7>0
union all
select count(*) from user_active_vv_20110801_31 where active_type_9>0
union all
select count(*) from user_active_vv_20110801_31 where active_type_11>0
union all
select count(*) from user_active_vv_20110801_31 where active_type_12>0
union all
select count(*) from user_active_vv_20110801_31 where active_type_17>0
union all
select count(*) from user_active_vv_20110801_31 where active_type_22>0;
报错：
FAILED: Error in semantic analysis: Top level UNION is not supported currently; use a subquery for the UNION
原来hive不支持顶层union，只能将union封装在子查询中；且必须为union的查询输出定义别名，正确的hql如下：
select * from (select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_3>0
UNION ALL
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_7>0
union all
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_9>0
union all
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_11>0
union all
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_12>0
union all
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_17>0
union all
select count(*) as type3 from user_active_vv_20110801_31 where user_active_vv_20110801_31.active_type_22>0) tmp;
执行结果如下：
原文：http://www.cnblogs.com/judylucky/p/3714001.html