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:
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: