Обсуждение: First Saturday and Last Saturday of a month

Hello all,

I have a Function that gives me the fsunday of a week Based on the date you
give it the select looks like
SELECT Date('now') - timespan( text( date_part( 'dow', Date('now') ) ) ||
'days' )
and if you run that today you will get
       ?column?
------------------------
  2001-08-05 00:00:00-04
(1 row)

Which is what I want it to do ...

Now I need to setup a function that will give me the first Saturday of the
month and then one to get the Last Saturday of the month.

I know that sounds odd but it is necessary for pulling information out of a
database.


Thank you all for you help.

Brian

"Brian C. Doyle" wrote:

> Hello all,
>
> I have a Function that gives me the fsunday of a week Based on the date you
> give it the select looks like
> SELECT Date('now') - timespan( text( date_part( 'dow', Date('now') ) ) ||
> 'days' )
> and if you run that today you will get
>        ?column?
> ------------------------
>   2001-08-05 00:00:00-04
> (1 row)
>
> Which is what I want it to do ...
>
> Now I need to setup a function that will give me the first Saturday of the
> month and then one to get the Last Saturday of the month.

It's too late to hack code, but:

For the first Saturday problem assign

    select date_part('dow', 'yyyy-mm-01'::DATE);

to a variable x.  Here yyy and mm is the year and month you are interested in.  The day you want is 'yyyy-mm-01' +
[(6-x)days]. 

Getting all the conversions right is left as an exercise for the reader :-)


Last Saturday problem is similar to above except for some sign reversals, but more interesting as you need the number
ofdays in the month.  I think you'll have to calculate that (for February) the hard way :-P  You *do* know the
algorithmfor testing if year yyyy is a leap year, don't you?   ((yyyy%4) && !(yyyy%400)) 


Allan.

Allan Engelhardt <allane@cybaea.com> writes:
> Last Saturday problem is similar to above except for some sign
> reversals, but more interesting as you need the number of days in the
> month.

I'd be inclined to take the first day of the *next* month (relatively
easy to figure), and then back up to a Saturday using the 'dow' value
for that day.

            regards, tom lane

Tom Lane wrote:

> Allan Engelhardt <allane@cybaea.com> writes:
> > Last Saturday problem is similar to above except for some sign
> > reversals, but more interesting as you need the number of days in the
> > month.
>
> I'd be inclined to take the first day of the *next* month (relatively
> easy to figure), and then back up to a Saturday using the 'dow' value
> for that day.

I *knew* there had to be a better way, even if I couldn't think of it late last night :-)

Thanks, Tom.

From: "Allan Engelhardt" <allane@cybaea.com>
To: "Brian C. Doyle" <bcdoyle@mindspring.com>;
<pgsql-general@postgresql.org>
Sent: Wednesday, August 08, 2001 11:18 PM
Subject: [GENERAL] Re: First Saturday and Last Saturday of a month


> You *do* know the algorithm for testing if year yyyy is a leap year, don't
you?

> ((yyyy%4) && !(yyyy%400))

ITYM ((yyyy%4) && ( !(yyyy%400) || (yyyy%1000) ) - year 2000 was a leap
year, even though it was a multiple of 400, because it was a millenium.

Alex
--
Alex Page, IT Department, Solid State Logic
E-Mail: alex.page@solid-state-logic.com
Phone: +44 (0) 1865 842 300
Web: http://www.solid-state-logic.com



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Alex Page writes:

> From: "Allan Engelhardt" <allane@cybaea.com>
> To: "Brian C. Doyle" <bcdoyle@mindspring.com>;
> <pgsql-general@postgresql.org>
> Sent: Wednesday, August 08, 2001 11:18 PM
> Subject: [GENERAL] Re: First Saturday and Last Saturday of a month
>
>
> > You *do* know the algorithm for testing if year yyyy is a leap year, don't
> you?
>
> > ((yyyy%4) && !(yyyy%400))
>
> ITYM ((yyyy%4) && ( !(yyyy%400) || (yyyy%1000) ) - year 2000 was a leap
> year, even though it was a multiple of 400, because it was a millenium.

Actually it's

y % 4 = 0 and (y % 100 <> 0 or y % 400 = 0)

(SQL pedants would use mod(y,4) etc. instead.)

--
Peter Eisentraut   peter_e@gmx.net   http://funkturm.homeip.net/~peter