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