Обсуждение: Query based on date/time field
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. Thanks, Will McCracken
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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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:
> 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
>
>
>