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https://issues.apache.org/jira/browse/HIVE-3577?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13856674#comment-13856674
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Ashutosh Chauhan commented on HIVE-3577:
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Support for 2 and 3 has landed in trunk via via HIVE-784
> Subquery Unnesting
> ------------------
>
> Key: HIVE-3577
> URL: https://issues.apache.org/jira/browse/HIVE-3577
> Project: Hive
> Issue Type: Sub-task
> Affects Versions: 0.10.0, 0.11.0, 0.12.0, 0.13.0
> Reporter: Shane Huang
> Priority: Critical
> Labels: SQL
>
> Hive requires subquery to be used only in the FROM clause. But real-world
> queries can be much more complex and subqueries could appear in WHERE, having
> and SELECT clauses. Many of such subqueries can be transformed/unnested to
> various join operations. This is an umbrella id to include all subquery
> unnesting work, specifically:
> 1. subquery in WHERE conditions, including subquery used as operand of IN,
> EXISTS, ISNULL ALL/ANY/SOME, and etc.
> 2. correlated subquery (a correlated subquery is a subquery that refers to a
> column of a table not in its FROM clause),e.g.
> {code}
> select * from t1 where exists ( select * from t2 where t1.b = t2.y );
> {code}
> 3. scalar subquery (a subquery that returns exactly one column value from one
> row). e.g.
> {code}
> select a,b,c,d,e,(select z from t2 where t2.y = t1.b and z != 99 ) from t1;
> {code}
> 4. top-level subquery, e.g.
> {code}
> (select * from t1) union all (select * from t2) union (all select * from t3
> order by 1);
> {code}
>
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