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From "John Sichi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1395) Table aliases are ambiguous
Date Thu, 24 Jun 2010 17:38:50 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1395?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12882235#action_12882235

John Sichi commented on HIVE-1395:

In the SQL standard, aliases are allowed to be reused at different levels of query nesting;
conflicts are only illegal in the same FROM clause.  This is the same rule as for column names,
actually (they can be reused at different levels of the query nesting, but not at the same
level).  There are corresponding rules for resolving references when correlated subqueries
are used (looking up into the closest scope).

I'm fine with making the rules stricter for Hive, since reusing a table alias is very confusing;
just pointing this out.

> Table aliases are ambiguous
> ---------------------------
>                 Key: HIVE-1395
>                 URL: https://issues.apache.org/jira/browse/HIVE-1395
>             Project: Hadoop Hive
>          Issue Type: Bug
>          Components: Query Processor
>            Reporter: Adam Kramer
> Consider this query:
> SELECT a.num FROM (
>   SELECT a.num AS num, b.num AS num2
>   FROM foo a LEFT OUTER JOIN bar b ON a.num=b.num
> ) a
> WHERE a.num2 IS NULL;
> ...in this case, the table alias 'a' is ambiguous. It could be the outer table (i.e.,
the subquery result), or it could be the inner table (foo).
> In the above case, Hive silently parses the outer reference to a as the inner reference.
The result, then, is akin to:
> SELECT foo.num FROM foo WHERE bar.num IS NULL. This is bad.
> The bigger problem, however, is that Hive even lets people use the same table alias at
multiple points in the query. We should simply throw an exception during the parse stage if
there is any ambiguity in which table is which, just like we do if the column names are ambiguous.
> Or, if for some reason we need people to be able to use 'a' to refer to multiple tables
or subqueries, it would be excellent if the exact parsing structure were made clear and added
to the wiki. In that case, I will file a separate bug JIRA to complain about how it should
be different. :)

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