Re: [SQL] (Fwd) Hi and some advice on Dates

Andrew Mather <mathera(at)woody(dot)agvic(dot)gov(dot)au> wrote:

> How to query a table and look for any records where the value in a
> date field is greater than (say) 10 days ago

Maybe like this:

-- create a(n example) table:
=> create table xxx (d date, something text);
CREATE
=> insert into xxx values ('09.11.1998', 'bbbbb!');
INSERT 72800 1
=> insert into xxx values ('02.11.1998', 'aaaa!');
INSERT 72801 1
=> insert into xxx values ('08.11.1998', 'cccc!');
INSERT 72802 1
-- have a look (note that I'm using the 'German' date style)
=> select * from xxx;

d|something
----------+---------
09.11.1998|bbbbb!
02.11.1998|aaaa!
08.11.1998|cccc!
(3 rows)

'today' gives current date when casted ('::' operator) to 'date' type;
the calculation of difference is simple:

=> select * from xxx
where d between 'today'::date - 10 and 'today'::date;

d|something
----------+---------
02.11.1998|aaaa!
08.11.1998|cccc!
(2 rows)

> Calculate the difference between two times on the same day. (I have
> experimented with age() and it sort of does wht I want..is there any
> other way ?)

There are various possibilities, depends on what you want. For
calculating another 'datetime', try:

=> select 'now'::datetime as now,
'now'::datetime - '10 secs'::timespan as ten_s_ago;

now |ten_s_ago
--------------------------+--------------------------
08.11.1998 09:59:54.00 MET|08.11.1998 09:59:44.00 MET
(1 row)

You have to cast to a proper type; e.g., you can't do

WRONG! => select 'now'::time as now,
WRONG! 'now'::time - '10 secs'::timespan as ten_s_ago;

probably because the 'time'-type doesn't know anything about the fact
that midnight is the beginning of a new day and everything is set to
zero ...

For calculating a 'timespan', do

=> select 'now'::datetime - '08.11.1998 06:00'::datetime as interv;

interv
------------------------
@ 4 hours 6 mins 30 secs(1 row)

or

=> select date_part('epoch',
'now'::datetime - '08.11.1998 06:00'::datetime)
as interv;
interv
-------
7708135
(1 row)

if you need the result in seconds.

Hope it helps!
Ulf

--
======================================================================
%%%%% Ulf Mehlig <ulf(dot)mehlig(at)uni-bremen(dot)de>
%%%%!%%% Projekt "MADAM" <umehlig(at)uni-bremen(dot)de>
%%%% %!% %%%% ----------------------------------------------------
---| %%% MADAM: MAngrove | Center for Tropical Marine
||--%!% Dynamics | Biology
|| And | Fahrenheitstrasse 1
_ /||\_/\_ Management |
/ / \ \ ~~~~~~~~~~~~~~~~~ | 28359 Bremen/Germany
~~~~~~~~~~~~~~~~~~~~