Query based on date/time field

At 15:50 +0300 on 12/8/98, Holger Mitterwald wrote:

On Tue, 4 Aug 1998, William McCracken wrote:

I am new to SQL and was wondering how I would go
about selecting records from my database based on
the age of a date/time stamp.

For example if I wanted to select records older than
12 hours or 1 day.

Any help would be appreciated.

Sorry for my late response, but I was on holiday.
I use some statement like this to select entries that are not older than 2
weeks:

SELECT DISTINCT landkreis FROM kneipe WHERE 1209600 >= (date_part('epoch',
current_date) - date_part('epoch',datum));

datum is of type datetime.

what I do is the following: epoch returns the seconds since january 1st
1970 (I think). I substract the timestamp of each record from the current
timestamp (current_date). The result has to be smaller than the duration
for 2 weeks (in seconds, which is 1209600s).

I dont know if there is a better solution, but it works fine.

I think the following is simpler and more intuitive:

SELECT * FROM the_table
WHERE the_date >= ('now'::datetime - '12 hours'::timespan);

This assumes that the field "the_date" in "the_table" is of type datetime.
If it isn't, just convert it by using datetime( the_date ).

The amount of time described by the type timespan is very intuitive - it
accepts, as you see, things like '12 hours', '1 day', or '2 months'. It
takes months correctly - with longer and shorter months taken into
accounts. More about it in the pgbuiltin manpage.

Herouth

--
Herouth Maoz, Internet developer.
Open University of Israel - Telem project
http://telem.openu.ac.il/~herutma

I just wanted to answer the initial question, but Heruoth managed to do so
before me:-) Anyway, I decided to play with datetime and timespan types
and was stuck with strange problem. Here it is:
If I subtract one datetime from another everithing is going as expected:
userbase=> select '12 Jun 98'::datetime-'11 Jun 98'::datetime;
?column?
--------
@ 1 day
(1 row)

But, subtracting one date from another one I got something like int:
userbase=> select '12 Jun 98'::date-'11 Jun 98'::date;
?column?
--------
1
(1 row)

The next query explains more clearly that type of result is "int4":
userbase=> select ('12 Jun 98'::date-'11 Jun 98'::date)::datetime;
ERROR: function datetime(int4) does not exist

Is that what is expected to be or a bug??
( I'm running PostgreSQL 6.3.2p1-4 on Linux (SlackWare) 2.0.34 )

On Wed, 12 Aug 1998, Herouth Maoz wrote:

On Tue, 4 Aug 1998, William McCracken wrote:

I am new to SQL and was wondering how I would go
about selecting records from my database based on
the age of a date/time stamp.

For example if I wanted to select records older than
12 hours or 1 day.

Any help would be appreciated.

Sorry for my late response, but I was on holiday.
I use some statement like this to select entries that are not older than 2
weeks:

SELECT DISTINCT landkreis FROM kneipe WHERE 1209600 >= (date_part('epoch',
current_date) - date_part('epoch',datum));

datum is of type datetime.

what I do is the following: epoch returns the seconds since january 1st
1970 (I think). I substract the timestamp of each record from the current
timestamp (current_date). The result has to be smaller than the duration
for 2 weeks (in seconds, which is 1209600s).

I dont know if there is a better solution, but it works fine.

So long,
Holger

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