Re: Extract date from week

On 11/8/05, lucas(at)presserv(dot)org <lucas(at)presserv(dot)org> wrote:
> Hi
> Looking the e-mail I remembered a question.
> I saw that "select extract (week from now()::date)" will return the
> week number
> of current year. But, how can I convert a week to the first reference
> date. Ex:
> select extract(week from '20050105'::date); -- 5 Jan 2005
> --Returns--
> date_part |
> 1 |
>
> It is the first week of year (2005), and how can I get what is the first date
> references the week 1? Ex:
> select ???? week 1
> --should return---
> date |
> 20050103 | -- 3 Jan 2005
>
> Thank you.
> Lucas Vendramin
>
>

Extracted from:
http://www.postgresql.org/docs/8.0/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT

--- begin extracted text ---

week
The number of the week of the year that the day is in. By definition
(ISO 8601), the first week of a year contains January 4 of that year.
(The ISO-8601 week starts on Monday.) In other words, the first
Thursday of a year is in week 1 of that year. (for timestamp values
only)

Because of this, it is possible for early January dates to be part of
the 52nd or 53rd week of the previous year. For example, 2005-01-01 is
part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd
week of year 2005.

SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 7

--- end extracted text ---

--
regards,
Jaime Casanova
(DBA: DataBase Aniquilator ;)