Thread: [GENERAL] 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@ludwig.ucl.ac.uk |
+-------------------------+--------------------------------------+
Re: [GENERAL] Typecasting datetype as date. How do I cope with NULLs?
From
tolik@icomm.ru (Anatoly K. Lasareff)
Date:
>>>>> "SR" == Stuart Rison <stuart@ludwig.ucl.ac.uk> writes:
SR> Dear All,
SR> For those of you who don't want to wade through the details, here's the
SR> question: "How do I get the date portion of a datetime field for ALL
SR> ENTRIES in a table regardless of whether the entry is NULL or not? (N.B.
SR> Typecasting a datetime NULL as date generates an error)"
. . .
SR> I find myself having to do two selects to get all the people in the table.
SR> i.e.:
SR> patients=> SELECT surname,firstname,othernames,dob::date FROM patients
SR> WHERE dob
SR> IS NOT NULL;
SR> surname|firstname|othernames | date
SR> -------+---------+--------------------+----------
SR> Goose |Mother |Lay Golden Eggs |11-01-1923
SR> One |Un |Uno Ein |11-11-1111
SR> Light |Dee |Full |22-01-1933
SR> (3 rows)
SR> patients=> SELECT surname,firstname,othernames FROM patients WHERE dob IS NULL;
SR> surname|firstname|othernames
SR> -------+---------+----------
SR> Rison |Stuart |
SR> Rison |This |Pal
SR> Rison |Mark |
SR> (3 rows)
SR> My question is, how do I get surname,firstname,othername and the date
SR> portion of ALL people in table people regardless of whether the entry has
SR> an actual dob or a NULL dob.
There are, on my mind, at least two answers. For experience I use
small table 'create table a( dt datetime, i int)'. Hera are data in
this table (one row has NULL as dt value):
tolik=> select * from a;
dt | i
----------------------------+--
Thu Nov 26 16:35:23 1998 MSK| 1
Wed Nov 25 00:00:00 1998 MSK| 2
Fri Nov 27 00:00:00 1998 MSK| 3
|10
First use 'union':
-----------------------------------------------
select dt::date, i from a where dt is not null
union
select NULL, i from a where dt is null;
date| i
----------+--
11-25-1998| 2
11-26-1998| 1
11-27-1998| 3
|10
(4 rows)
-----------------------------------------------
Second, try use date_trunc('day', dt) instead date_part:
--------------------------------------------------------------
tolik=> select date_trunc('day', dt), i from a;
date_trunc | i
----------------------------+--
Thu Nov 26 00:00:00 1998 MSK| 1
Wed Nov 25 00:00:00 1998 MSK| 2
Fri Nov 27 00:00:00 1998 MSK| 3
|10
(4 rows)
--------------------------------------------------------------
Regards!
--
Anatoly K. Lasareff Email: tolik@icomm.ru
Senior programmer
>There are, on my mind, at least two answers. For experience I use
>small table 'create table a( dt datetime, i int)'. Hera are data in
>this table (one row has NULL as dt value):
>
>tolik=> select * from a;
>dt | i
>----------------------------+--
>Thu Nov 26 16:35:23 1998 MSK| 1
>Wed Nov 25 00:00:00 1998 MSK| 2
>Fri Nov 27 00:00:00 1998 MSK| 3
> |10
>
>First use 'union':
>-----------------------------------------------
>select dt::date, i from a where dt is not null
>union
>select NULL, i from a where dt is null;
> date| i
>----------+--
>11-25-1998| 2
>11-26-1998| 1
>11-27-1998| 3
> |10
>(4 rows)
>-----------------------------------------------
>
>
>Second, try use date_trunc('day', dt) instead date_part:
>--------------------------------------------------------------
>tolik=> select date_trunc('day', dt), i from a;
>date_trunc | i
>----------------------------+--
>Thu Nov 26 00:00:00 1998 MSK| 1
>Wed Nov 25 00:00:00 1998 MSK| 2
>Fri Nov 27 00:00:00 1998 MSK| 3
> |10
>(4 rows)
>
>--------------------------------------------------------------
>
I find the above interesting and I've tried something similar that won't
work.
I have two tables defined as follows
create table test1 (id1 int,link1 int);
create table test2 (id2 int, field2 varchar(5));
where link1 is a foreign key of test1 that should be linked to id2 of test2
Now when I execute the following query:
select id1,field2,link1 from test1,test2 where test1.link1=test2.id2
union
select id1,NULL,link1 from test1;
I always get the following error:
Each UNION query must have identical target types.
Why this error, and what does it mean?
TIA
Wim Ceulemans - wim.ceulemans@nice.be
Nice Software Solutions
Eglegemweg 3, 2811 Hombeek - Belgium
Tel +32(0)15 41 29 53 - Fax +32(0)15 41 29 54
Re: [GENERAL] Typecasting datetype as date. How do I cope with NULLs?
From
tolik@icomm.ru (Anatoly K. Lasareff)
Date:
>>>>> "WC" == Wim Ceulemans <wim.ceulemans@nice.be> writes: WC> I find the above interesting and I've tried something similar that won't WC> work. WC> I have two tables defined as follows WC> create table test1 (id1 int,link1 int); WC> create table test2 (id2 int, field2 varchar(5)); WC> where link1 is a foreign key of test1 that should be linked to id2 of test2 WC> Now when I execute the following query: WC> select id1,field2,link1 from test1,test2 where test1.link1=test2.id2 WC> union WC> select id1,NULL,link1 from test1; WC> I always get the following error: WC> Each UNION query must have identical target types. WC> Why this error, and what does it mean? Pehaps this is a bug fixed in 6.4 version? I saw this message when before upgrating to 6.4. You can modify query so: select id1,field2,link1 from test1,test2 where test1.link1=test2.id2 union select id1, -1, link1 from test1; where -1 is value non present in field2 domain. -- Anatoly K. Lasareff Email: tolik@icomm.ru Senior programmer
> SR> Dear All,
> SR> For those of you who don't want to wade through the details, here's the
> SR> question: "How do I get the date portion of a datetime field for ALL
> SR> ENTRIES in a table regardless of whether the entry is NULL or not? (N.B.
> SR> Typecasting a datetime NULL as date generates an error)"
>
<snip>
>
>There are, on my mind, at least two answers. For experience I use
>small table 'create table a( dt datetime, i int)'. Hera are data in
>this table (one row has NULL as dt value):
>
>tolik=> select * from a;
>dt | i
>----------------------------+--
>Thu Nov 26 16:35:23 1998 MSK| 1
>Wed Nov 25 00:00:00 1998 MSK| 2
>Fri Nov 27 00:00:00 1998 MSK| 3
> |10
>
>First use 'union':
>-----------------------------------------------
>select dt::date, i from a where dt is not null
>union
>select NULL, i from a where dt is null;
> date| i
>----------+--
>11-25-1998| 2
>11-26-1998| 1
>11-27-1998| 3
> |10
>(4 rows)
I had not thought of using a UNION, thanks for that (well that's not
strictly true, I was 'procedurally' implemeting it rather then using SQL!!)
so this is a definite possibility.
I still feel that this equates to two SQL queries since the backend will
have to process each individually and then UNION them (is this right?). My
suggestion of:
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)
means only one query need to be executed by the backend and it can cope
with NULL, but it 'generates' three date fields and I would like only one.
>
>Second, try use date_trunc('day', dt) instead date_part:
>--------------------------------------------------------------
>tolik=> select date_trunc('day', dt), i from a;
>date_trunc | i
>----------------------------+--
>Thu Nov 26 00:00:00 1998 MSK| 1
>Wed Nov 25 00:00:00 1998 MSK| 2
>Fri Nov 27 00:00:00 1998 MSK| 3
> |10
>(4 rows)
>
>--------------------------------------------------------------
Again, this solves one part of my problem (i.e. the query functions even if
I datetime is NULL) but what I'm looking for is something that ONLY shows
the date portion of a datetime (e.g. Thu Nov 26 16:35:23 1998 MSK becomes
26-11-1998) so the date_trunc masking is not quite what I'm looking for.
Would it be possible to do a select datetime and then output only the
dateday, datemonth and dateyear using an RE??
All the same, thanks for your suggestion Anatoly, UNION's the winner so far ;)
regards,
S.
+-------------------------+--------------------------------------+
| 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@ludwig.ucl.ac.uk |
+-------------------------+--------------------------------------+
At 13:12 +0200 on 27/11/98, Stuart Rison wrote: > > Again, this solves one part of my problem (i.e. the query functions even if > I datetime is NULL) but what I'm looking for is something that ONLY shows > the date portion of a datetime (e.g. Thu Nov 26 16:35:23 1998 MSK becomes > 26-11-1998) so the date_trunc masking is not quite what I'm looking for. > > Would it be possible to do a select datetime and then output only the > dateday, datemonth and dateyear using an RE?? > > All the same, thanks for your suggestion Anatoly, UNION's the winner so >far ;) Well, first I'd like to say that not converting null datetimes is definitely a bug and should be placed in the ToDo list for the next version. That said, I tried several conversions, using the following table: testing=> select * from test1; f1 |dt ------------+---------------------------- This year |Fri Jan 02 00:00:00 1998 IST Next year |Mon Mar 15 00:00:00 1999 IST Nothing | Last year |Mon May 12 00:00:00 1997 IDT More nothing| (5 rows) The one that finally worked is the following: testing=> select f1, date( abstime( dt ) ) from test1; f1 | date ------------+---------- This year |01-02-1998 Next year |03-15-1999 Nothing | Last year |05-12-1997 More nothing| (5 rows) That is, convert to abstime, and then to date... Herouth -- Herouth Maoz, Internet developer. Open University of Israel - Telem project http://telem.openu.ac.il/~herutma