两个查询的单元单元联合出现编译错误

我不确定下面的配置单元查询出了什么问题。然而,这是一个投掷错误。

select h.database_name, h.table_name, h.frequency as hive_frequency, i.frequency as impala_frequency    
from hive_access_frequency h
left join impala_access_frequency i
on 
h.database_name = i.database_name and 
h.table_name = i.table_name
union
select i.database_name, i.table_name, h.frequency as hive_frequency, i.frequency as impala_frequency    
from impala_access_frequency i
left join hive_access_frequency h
on 
h.database_name = i.database_name and 
h.table_name = i.table_name

我可以单独运行这些查询。但是,在与 union

错误

Error: Error while compiling statement: FAILED: ParseException line 8:0 missing ALL at 'select' near '<EOF>' (state=42000,code=40000)