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)