More on unique vs distinct

I guess I should have been a bit more specific.  Here is an example of
the type of query I'm finding in the Oracle app and the results of my
attempt to find  a PostgreSQL equivalent.

select count(unique 'source_id') from obis_sources;
pgAdmin III query responds:
ERROR:  syntax error at or near "'source_id'" at character 23

select count(distinct 'source_id') from obis_sources;
pgAdmin III query responds:
ERROR:  could not identify an equality operator for type "unknown"

Thanks for all the help,
-=bill