union — Returns an union of two or more sets
sequence union (
|set1 sequence ,
|set2 sequence ... ,
The function returns an unordered sequence that consists of all distinct items from all given sets.
All duplicates are removed, including duplicate occurrences of
same value in one sequence, thus the function works exactly as
distinct() when only
one parameter is passed.
Sequence that contains items of the resulting union.
The function returns an unordered sequence of distinct values.