"left join" not working?

Hi,

This query:

select c.id_currency,max(p.modified_on) from currency c left join
price_line p using (id_currency) where p.id_line=1 group by
c.id_currency;

doesn't list all c.id_currency's, only those with a price_line. However
this one does:

select c.id_currency,max(p.modified_on) from currency c left join
price_line p on (p.id_currency = c.id_currency and p.id_line=1) group by
c.id_currency;

How come?

Thanks,