how to constrain a query to return 1 or 0 rows (or >1 or 0 rows)

This is probably a stupid question, but ...

I'd like to be able to take an existing query and modify it to return a
single row if that's what the base query returns, and 0 rows if the
base query returns multiple rows. Similarly, I'd like to also modify
it to return multiple rows if that's what the base query returns, and 0
rows if the base query return a single row.

What's a good way to do this?

Thanks,
Kevin Murphy