How can I get the first and last date of a week, based on the week number and the year

On Tuesday 26 February 2008 5:32 pm, Bruno Baguette wrote:

Hello !

I have a week number (ISO 8601) and a year, based on theses values, I
would like to get the first and the last dates of that week.

How I can do that ?

The only solution is doing a big generate_series to build a subset that
contains the week of all the dates between the 01/01 || year and the
31/12 || year. But I find that solution quite dirty and ressources
consumming.

Is there a cleanest way to do that ?

Many thanks in advance !

Regards,

This will get you to the Monday of the week .

select to_date('9 08','IW YY');
to_date
------------
2008-02-25
(1 row)

Where 9 is the ISO week number and 08 is the year. See below for more details:
http://www.postgresql.org/docs/8.2/interactive/functions-formatting.html
--
Adrian Klaver
aklaver@comcast.net

Bruno Baguette wrote:

Hello !

I have a week number (ISO 8601) and a year, based on theses values, I
would like to get the first and the last dates of that week.

How I can do that ?

The only solution is doing a big generate_series to build a subset that
contains the week of all the dates between the 01/01 || year and the
31/12 || year. But I find that solution quite dirty and ressources
consumming.

Is there a cleanest way to do that ?

Use pl/Perl and the Date::Calc module.

Adrian Klaver <aklaver@comcast.net> writes:

On Tuesday 26 February 2008 5:32 pm, Bruno Baguette wrote:

I have a week number (ISO 8601) and a year, based on theses values, I
would like to get the first and the last dates of that week.

This will get you to the Monday of the week .

select to_date('9 08','IW YY');
to_date
------------
2008-02-25
(1 row)

date_trunc('week', ...) would probably be a more convenient way of doing
that.

regards, tom lane

On Tuesday 26 February 2008 8:39 pm, Tom Lane wrote:

Adrian Klaver <aklaver@comcast.net> writes:

On Tuesday 26 February 2008 5:32 pm, Bruno Baguette wrote:

I have a week number (ISO 8601) and a year, based on theses values, I
would like to get the first and the last dates of that week.

This will get you to the Monday of the week .

select to_date('9 08','IW YY');
to_date
------------
2008-02-25
(1 row)

date_trunc('week', ...) would probably be a more convenient way of doing
that.

regards, tom lane

I am not sure I follow. The OP has a ISO week number and a year and wants
dates. I thought date_trunc('field',source) requires a timestamp or interval
as its source.
--
Adrian Klaver
aklaver@comcast.net

Adrian Klaver <aklaver@comcast.net> writes:

On Tuesday 26 February 2008 8:39 pm, Tom Lane wrote:

date_trunc('week', ...) would probably be a more convenient way of doing
that.

I am not sure I follow. The OP has a ISO week number and a year and wants
dates. I thought date_trunc('field',source) requires a timestamp or interval
as its source.

Oh, you are right ... I misread the question :-(

regards, tom lane

Bruno Baguette wrote:

Hello !

I have a week number (ISO 8601) and a year, based on theses values, I
would like to get the first and the last dates of that week.

Broken down step-by-step. End of week left as an exercise

SELECT
doy,
EXTRACT(dow FROM doy) AS offset,
(doy - EXTRACT(dow FROM doy) * '1 day'::interval)::date AS start_of_week
FROM
(SELECT ('2008-01-04'::date + 8 * '1 week'::interval)::date AS doy)
AS foo;

doy | offset | start_of_week
------------+--------+---------------
2008-02-29 | 5 | 2008-02-24
(1 row)

--
Richard Huxton
Archonet Ltd

On Feb 27, 2008, at 2:32 AM, Bruno Baguette wrote:

Hello !

I have a week number (ISO 8601) and a year, based on theses values,
I would like to get the first and the last dates of that week.

How I can do that ?

The only solution is doing a big generate_series to build a subset
that contains the week of all the dates between the 01/01 || year
and the 31/12 || year. But I find that solution quite dirty and
ressources consumming.

Is there a cleanest way to do that ?

You can use to_date for most of that, like:
development=> select to_date('01 02 2008', 'ID IW YYYY') AS start,
to_date('07 02 2008', 'ID IW YYYY') AS end;
start | end
------------+------------
2008-01-07 | 2008-01-07
(1 row)

I'm a bit surprised that specifying the weekdays doesn't make any
difference here, maybe it's my version?:

development=> select version();
version
------------------------------------------------------------------------
------------------------
PostgreSQL 8.2.4 on i386-portbld-freebsd6.2, compiled by GCC cc
(GCC) 3.4.6 [FreeBSD] 20060305
(1 row)

Anyway, you can solve that by adding an interval '6 days' to the end
result.

Alban Hertroys

--
If you can't see the forest for the trees,
cut the trees and you'll see there is no forest.

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