Counts and percentages and such

I'm new to both pgsql and SQL in  general pas really simple stuff, so
i would like to know how to;

Given a table with a column that can have one of NULL, (char) N,
(char) A, and (char) L. Is there a way to in a single query, ge the
percentage of the whole rowset that each of those represents?

like :

75% Null
15% A
5% N
5% L