It's a bit dangerous to visualize Union like that, people might confuse it with concatenation which it's not. You'll want to use "UNION ALL" if your intent is

It's a bit dangerous to visualize Union like that, people might confuse it with concatenation which it's not. You'll want to use "UNION ALL" if your intent is to concatenate the two sets of rows, and not to calculate the (deduplicated) union. If only because it's much faster if your database doesn't have to check for duplicates.