Hadoop Hive Not recognizing alias named in select part

Question

When I invoke hql as follows, it tells me that it cannot find alias for u1.

hive> select user as u1, url as u2 from rank_test where u1 != "";
FAILED: SemanticException [Error 10004]: Line 1:50 Invalid table alias or column reference 'u1': (possible column names are: user, url)

This problem is the same as when I try to use count(*) as cnt. Could anyone give me some hint on how to use alias in where clause? Thanks a lot!

hive> select user, count(*) as cnt from rank_test where cnt >= 2 group by user;
FAILED: ParseException line 1:58 missing EOF at 'where' near 'user'

Answer 1

The where clause is evaluated before the select clause, which is why you can't refer to select aliases in your where clause.

You can however refer to aliases from a derived table.

select * from (
  select user as u1, url as u2 from rank_test
) t1 where u1 <> "";

select * from (
  select user, count(*) as cnt from rank_test group by user
) t1 where cnt >= 2;

Side note: a more efficient way to write the last query would be

select user, count(*) as cnt from rank_test group by user
having count(*) >= 2

Answer 2

You can not use column aliases in where clause predicates.

Comments

I have a doubt. Imagine there are two companies - A and B. Now company B has the need to access the database of company A. For security reasons, it can give the actual table name but gives an alias name. How will the company B access the database if it cannot use the alias name in the where clause?