Range

Hi All,

I am writing a query that UNIONs a bunch of data together.  I would like to add
a query to the UNION that will insure that if nothing else is retrieved a zero
will be available.

The data I am retrieving in each part of the UNION looks something like this
item,quantity,ship_date
A,   10,      2006-01-01
A,    5,      2006-02-01
A,    3,      2006-02-01
A,   20,      2006-04-01

Notice the gap in the date sequence.

How can I generate a zero quantity to fill the gap?  Either in the original
retrieval or as a seperate query for the UNION or some other way so that when I
GROUP BY the date and SUM the quantity I end up with

item,quantity,ship_date
A,   10,      2006-01-01
A,    8,      2006-02-01
A,    0,      2006-03-01
A,   20,      2006-04-01

The queries whose results are being assembled with the UNION all have the form
SELECT item,
       quantity,
       DATE_TRUNC('MONTH', source_table.ship_date)::date AS ship_date
  FROM source_table;

As always hints, suggestions and URLs for applicable documentation will be
appreciate.

Kind Regards,
Keith