[SQL] counting distinct values

[Joseph Shraibman]

Using the example from
http://www.postgresql.org/docs/aw_pgsql_book/node59.html, what would I
do if I wanted to know the number of different cities where I had a
friend in each state?  select count(city) group by state; would not work
because if you had two friends in the same city it would be counted
twice.