MySQL: Finding the records in one table that are not in another table

OK, this’ll make users of “proper databases” cringe but because MySQL doesn’t support sub-selects you can’t do the most obvious way of selecting records in one table that aren’t in another. Here’s how it’s done, with a clear explanation of why it works!
from po left join pn
where is null

Unfortunately, I can’t figure out how to filter the second table as I don’t want to select from the whole table, only a subset. Anyone solved this?