UPDATE comparing dates and non-dates data

Hi;
I've got date data, extracted from rain gauge stations. The date of a row in
my data are structured in three integer fields, and as a result of a query I
can get the following (what is quite good for my):
cod_station | year | month | day | rain
-------------+------+-------+-----+------
7250 | 1933 | 8 | 1 | 45
7250 | 1933 | 8 | 2 | 3
7250 | 1933 | 8 | 3 | 0
...
and this for several rain station for about forty years. My problem is that I
need to find missing data (a missing data here is a non-existent row).

I think I could prepare a table with a complete series of date field, and
ALTER it to ADD aditional fields for every station I need to add:

date | stat_7250 | stat_7237 ...
----------------+--------------------+---------------+------...
1/11/1999
2/12/1999
...
, and make an UPDATE that in someway, will compare the "date" of this table
with an extracted date from "year", "day" and "rain" from the aforementioned
SELECT, leaving the non-coincident dates in blank and filling the rain data
of the coincident dates. Is this possible? How?

Please help.
Thank you for any possible advice

--
A. Javier García
Water and Soil Conservation Department
CEBAS-CSIC
Apartado 4195
30080 Murcia
Spain

Tel.: +34 968 39 63 90
Fax: +34 968 39 62 13