Since Ross gave you an answer for the second question, I try the first one.

Why don't you like using an IF? It could look like this:

IF EXISTS(SELECT * FROM current where ...)
  SELECT * FROM current where ...
ELSE
  SELECT * FROM history where ...  

If it wasn't a current or history situation, in other words the query might find rows in both tables, you could do something like this:

SELECT * FROM current WHERE ...
UNION
SELECT * FROM history WHERE ...

-Mike