[SQL] Typecasting datetype as date. How do I cope with NULLs?

Dear All,

For those of you who don't want to wade through the details, here's the
question: "How do I get the date portion of a datetime field for ALL
ENTRIES in a table regardless of whether the entry is NULL or not? (N.B.
Typecasting a datetime NULL as date generates an error)"

details...

I have a table which stores, among other information, people's Date of
Birth as a datetime datatype called dob (I don't use the 'time' part but
the datatype is supported by many more functions). Of course, I don't
always know the birthday of the person involved so the filed sometimes has
a NULL value.

patients=> \d patients

Table = patients
+----------------------------------+----------------------------------+-------+
| Field | Type | Length|
+----------------------------------+----------------------------------+-------+
| patient_id | char() not null | 16 |
| surname | text | var |
| firstname | text | var |
| othernames | text | var |
| dob | datetime | 8 |
| sex | char() | 1 |
+----------------------------------+----------------------------------+-------+
Index: patients_new_pkey
patients=>

I have a select on this table which typecasts the dob (Date of Birth) as
date i.e.:

patients=> SELECT surname,firstname,othernames,dob::date FROM patients;
ERROR: Unable to convert null datetime to date
patients=>

I find myself having to do two selects to get all the people in the table.
i.e.:

patients=> SELECT surname,firstname,othernames,dob::date FROM patients
WHERE dob
IS NOT NULL;
surname|firstname|othernames | date
-------+---------+--------------------+----------
Goose |Mother |Lay Golden Eggs |11-01-1923
One |Un |Uno Ein |11-11-1111
Light |Dee |Full |22-01-1933
(3 rows)

patients=> SELECT surname,firstname,othernames FROM patients WHERE dob IS NULL;
surname|firstname|othernames
-------+---------+----------
Rison |Stuart |
Rison |This |Pal
Rison |Mark |
(3 rows)

My question is, how do I get surname,firstname,othername and the date
portion of ALL people in table people regardless of whether the entry has
an actual dob or a NULL dob.

The best I have managed so far is:

patients=> SELECT surname,firstname,othernames,date_part('day',dob) as dd,
patients-> date_part('month',dob) as mm, date_part('year',dob) as yyyy
patients-> FROM patients;
surname|firstname|othernames |dd|mm|yyyy
-------+---------+--------------------+--+--+----
Goose |Mother |Lay Golden Eggs |11| 1|1923
One |Un |Uno Ein |11|11|1111
Light |Dee |Full |22| 1|1933
Rison |Stuart | | | |
Rison |This |Pal | | |
Rison |Mark | | | |
(6 rows)

patients=>

But I would like to have the date of birth as one field rather than 3. I
have tried concatenating with || (no joy, date_part returns a float8 so
can't cat) and textcat (same problem). Typecasting the return value of
date_part does not work (e.g.:
patients=> select date_part('day',dob)::text ||
date_part('month',dob)::text fro
m patients;
?column?
--------------------------------------------------------
Sat 01 Jan 00:00:11 2000 GMTSat 01 Jan 00:00:01 2000 GMT
Sat 01 Jan 00:00:01 2000 GMTSat 01 Jan 00:00:04 2000 GMT
Sat 01 Jan 00:00:11 2000 GMTSat 01 Jan 00:00:11 2000 GMT
Sat 01 Jan 00:00:11 2000 GMTSat 01 Jan 00:00:11 2000 GMT
Sat 01 Jan 00:00:11 2000 GMTSat 01 Jan 00:00:11 2000 GMT
Sat 01 Jan 00:00:11 2000 GMTSat 01 Jan 00:00:11 2000 GMT

(6 rows)

patients=> select
textcat(date_part('day',dob)::text,date_part('month',dob)::text);
same output!

CAN IT BE DONE???

cheers,

Stuart.

+-------------------------+--------------------------------------+
| Stuart Rison | Ludwig Institute for Cancer Research |
+-------------------------+ 91 Riding House Street |
| Tel. (0171) 878 4041 | London, W1P 8BT, UNITED KINGDOM. |
| Fax. (0171) 878 4040 | stuart(at)ludwig(dot)ucl(dot)ac(dot)uk |
+-------------------------+--------------------------------------+