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https://issues.apache.org/jira/browse/HIVE-1395?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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John Sichi resolved HIVE-1395.
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Resolution: Won't Fix
We're fixing the bugs and sticking with the normal SQL rules, which allow
duplicate aliases, for the reasons mentioned above.
> Table aliases are ambiguous
> ---------------------------
>
> Key: HIVE-1395
> URL: https://issues.apache.org/jira/browse/HIVE-1395
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.6.0
> 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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