How to return a default value if no result

I want my query to return at least one row with a default result even if
there are no matches in the database.

As a simple example, getting the valid VAT rate for a transaction date and
VAT code:

Select vat_rate from vatrates where vat_code='1' and vat_date<='2014-08-01'
order by vat_date desc limit 1

This is fine as long as there is a match. If there are no matches, I'd like
it to return a default (0) rather than no result.

I could make sure there is always a result by having a UNION with a
hard-coded record, but this doesn't let me order by a field not in the
result.

In this example I could just make sure there is a dummy row in the table
with a very old date and do something like this:

Select vat_rate from vatrates where (vat_code='1' or vat_code='DUMMY') and
vat_date<='2014-08-01' order by vat_date desc limit 1

But it seems like a bodge to me.

Is there a nice way of forcing a default result somehow?

Thanks,

Rob