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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Trivial Update of "Hive/LanguageManual/LanguageManual/Union" by RaghothamMurthy
Date Wed, 21 Jan 2009 23:30:03 GMT
Dear Wiki user,

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The following page has been changed by RaghothamMurthy:
http://wiki.apache.org/hadoop/Hive/LanguageManual/LanguageManual/Union

------------------------------------------------------------------------------
+  
- Syntax:
- {{{
- select_statement UNION ALL select_statement UNION ALL select_statement ...
- }}}
  
- UNION is used to combine the result from multiple SELECT statements into a single result
set. We currently only support UNION ALL (bag union) i.e. duplicates are not eliminated. The
number and names of columns returned by each select_statement has to be the same. Otherwise,
a schema error is thrown.
- 
- If some additional processing has to be done on the result of the UNION, the entire statement
expression can be embedded in a FROM clause like below:
- {{{
- SELECT *
- FROM (
-   select_statement
-   UNION ALL
-   select_statement
- ) unionResult
- }}}
- 
- 
- For example, if we suppose there are two different tables that track which user has published
a video and which user has published a comment, the following query joins the results of a
union all with the user table to create a single annotated stream for all the video publishing
and comment publishing events:
- {{{
-     SELECT u.id, actions.date
-     FROM (
-         SELECT av.uid AS uid 
-         FROM action_video av 
-         WHERE av.date = '2008-06-03' 
-         UNION ALL 
-         SELECT ac.uid AS uid 
-         FROM action_comment ac 
-         WHERE ac.date = '2008-06-03' 
-      ) actions JOIN users u ON (u.id = actions.uid) 
- }}}
- 

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