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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.dateFROM ( 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.
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原文地址：http://www.cnblogs.com/xiejin/p/4249843.html
