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From: Michael Glaesemann <grzm@seespotcode.net>
Subject: Re: current_date / datetime stuff
Date: Mon, 4 Jun 2007 10:59:20 -0500
To: Andrew Sullivan <ajs@crankycanuck.ca>


On Jun 4, 2007, at 10:27 , Andrew Sullivan wrote:

> On Mon, Jun 04, 2007 at 10:04:37AM -0500, Joshua wrote:
>> that will return the date of the first Monday of the month?
>
> I guess you need to write a function to do this.  I suppose you could
> do it by finding out what day of the week it is and what the date is,
> then counting backwards to the earliest possible Monday.

As Andrew said, there's no built-in function to do this, but it's  
easy enough to write one. Here's a rough example (very lightly tested  
and probably overly complicated)

CREATE OR REPLACE FUNCTION first_dow(DATE, INTEGER)
RETURNS DATE
IMMUTABLE
LANGUAGE SQL AS $_$
SELECT v_first_day_of_month +
        CASE WHEN v_day_of_week <= $2 THEN $2 - v_day_of_week
             ELSE 8 - v_day_of_week
        END AS first_day_of_month
FROM (
      SELECT v_first_day_of_month
             , extract('dow' from v_first_day_of_month)::integer
                             AS v_day_of_week
      FROM (SELECT date_trunc('month', $1)::date)
           AS mon(v_first_day_of_month)) as calc;
$_$;

CREATE OR REPLACE FUNCTION first_monday(DATE)
RETURNS DATE
IMMUTABLE
LANGUAGE SQL AS $_$
SELECT first_dow($1, 1);
$_$;

select first_monday(current_date);
first_monday
--------------
2007-06-04
(1 row)

select first_monday('2007-04-01');
first_monday
--------------
2007-04-02
(1 row)

Michael Glaesemann
grzm seespotcode net