[HIve - LanguageManual] Union

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. Hive currently only supports UNION ALL (bag union), in which duplicates are not eliminated. The number and names of columns returned by each select_statement have 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)

Unions can be used in views, inserts, and CTAS (create table as select) statements. A query can contain multiple UNION ALL clauses, as shown in the syntax above.

Version information

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In Hive 0.12.0 and earlier releases, unions can only be used within a subquery such as "SELECT * FROM (select_statement UNION ALLselect_statement UNION ALL ...) unionResult".

As of Hive 0.13.0, unions can also be used in a top-level query: "select_statement UNION ALL select_statement UNION ALL ...". (See HIVE-6189.)