Hive如何一次查询多张表？

Question

I have to query and then "union" many tables. I did manually in Hive but wondering if there's a more optimal (shorter) way to do it.

We have tables for each month, so instead of doing this for a whole year:

create table t_2019 as
select * from
(select * from t_jan where...
union all
select * from t_feb where...
union all
select * from t_mar where...);

Does Hive (or any kind of SQL) allow to loop through tables? I've seen for loop and while examples in T-SQL, but they are individual queries. In this case I want to union the tables.

@t_list = ('t_jan', 't_feb', 't_mar'...etc)

Then, how to query each table in @t_list and "union all"? Each month has about 800k rows, so it's big but Hive can handle.

Answer 1

You can solve this problem with partitioned hive table instead of multiple tables.

Ex: table_whole pointing to hdfs path hdfs://path/to/whole/ with partitions on Year and Month

Now you can query to get data from  all months in 2019 using
select * from table_whole where year = '2019' 


If you need just data from one month say Jan in 2019. you can filter by that partition 
select * from table_whole where year = '2019' and month='JAN'