Inserting into table only if the row does not already exist.

Hey folks,
  I am inserting data into a table with a three-column primary key (the
table is only three columns).  The rows I'm trying to insert may very
well be duplicates of ones already in the table, so I would like to have
PostgreSQL handle the insert and possible error resulting from dup data.
I can certainly do a SELECT first and then INSERT if it's not a duplicate,
ala:

* Do a SELECT against the three columns
* If there are no rows returned, then do the INSERT
  But I thought this would be possible with the following:

INSERT INTO table ( column1, column2, column3 )  SELECT column1, column2, column3  WHERE NOT EXISTS (     SELECT
column1,column2, column3 FROM table WHERE        column1 = $column1 AND        column2 = $column2 AND        column3 =
$column3)
 
  .. which gave me 'ERROR: column1 does not exist'.  Nuts.
  Is this possible to do in one statement?  I checked with a DBA friend
(he's an Oracle guy), Google, and the list archives, and either didn't
find anything helpful or simply didn't try the correct search terms.  Or
should I be doing this sort of thing in two separate queries?

Thanks for all the help you folks have given me,

Benny


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