select distinct() question

I observed the following for one of my queries:

TEST=# select distinct(prod_id), date_received from invoice_li;
  prod_id | date_received
---------+---------------
    10994 | 2003-04-02
    12445 | 2003-04-08
    12445 |
(3 rows)


Is this the correct behaviour for a select distinct()?

I tried understanding what the SQL standard says but was thoroughly
confused ;)

Jean-Christian Imbeault