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Date: Mon, 15 Oct 2012 06:55:03 +0000 (UTC)
From: "Shengsheng Huang (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
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Subject: [jira] [Updated] (HIVE-3577) Subquery Unnesting
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     [ https://issues.apache.org/jira/browse/HIVE-3577?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Shengsheng Huang updated HIVE-3577:
-----------------------------------

    Description: 
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}

  
  was:
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)
{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. select a,b,c,d,e,(select z from t2 where t2.y = t1.b and z != 99 ) from t1;

4. top-level subquery
(select * from t1) union all (select * from t2) union (all select * from t3 order by 1);


> 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
>            Reporter: Shengsheng Huang
>
> 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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