Would that work in MySql? Back in the day it used to do things like "natural join" that just figured out the the common columns to join, but i can't remember if it did that with union.

NATURAL JOIN is actually in the SQL standard; it's not specific to MySQL.

I'm not aware of any databases that allow UNION to take table names directly, although in many cases you can use the TABLE keyword as a shortcut for "SELECT * FROM".