Union Vs Union All
Union operation combines two/more result tables that are created from the same base table. But the database performs an implicit DISTINCT operation. Where as in UNION ALL its just combines two/more result tables [ duplicate occurs].
The below screen shot shows two results tables LT_CARRID1 & LT_CARRID2 which contains exact same records.
The UNION of these two result tables gives back the combined distinct records where as the UNION ALL gives back the combined records.