Specifies the union of two data sources.

    Either a join or union method must be specified between all data
    sources with the exception of QUERY source blocks.

                                   NOTE

       When a join method appears immediately before an ordered
       or unordered group, the join method is associated with the
       first join item named in the group.

    The union strategy is only valid for queries that use the UNION
    operator, and all queries that specify the UNION operator must
    use the union strategy.