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Re: Date for a week day of a month

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On Jul 3, 2007, at 13:27 , Emi Lu wrote:

Can I know how to get the date of each month's last Thursday please?

For example, something like

Query:  select getDateBaseOnWeekday('2007-04-01', 'Last Thursday');
Result: 2007-04-26

Using a function for calculating the first occurrence of a particular day of the week in a month from a previous post[1], this should work. Natural language processing left as an exercise to the reader :)

CREATE OR REPLACE FUNCTION first_dow_of_month(DATE, INTEGER)
RETURNS DATE
IMMUTABLE
LANGUAGE SQL AS $_$
SELECT v_first_day_of_month + ( 7 + $2 - v_day_of_week) % 7
       AS first_dow_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;
$_$;
COMMENT ON FUNCTION first_dow_of_month(DATE, INTEGER) IS
'first_dow_of_month(date, integer) returns the first occurrence of a particular weekday in ' 'a given month. The first argument supplies the month (as a date), and the second '
'argument is the day of the week index as returned by extract(''dow'')';


CREATE OR REPLACE FUNCTION nth_dow_of_month (
       DATE -- date in target month
       , INTEGER -- day of week index
       , INTEGER -- zero-based ordinal day of week index,
                 -- e.g., 0 is first, 1 is second, -1 is last.
)
RETURNS DATE
IMMUTABLE
STRICT
LANGUAGE sql AS $_$
    SELECT CASE
        WHEN $3 >= 0 THEN
            first_dow_of_month($1, $2) + $3 * 7
        ELSE
first_dow_of_month(($1 + interval '1 month')::date, $2) + $3 * 7
        END;
$_$;
COMMENT ON FUNCTION nth_dow_of_month(DATE, INTEGER, INTEGER) IS
'nth_dow_of_month(date, integer, integer) returns the nth occurrence of a particular ' 'weekday in a given month. The first argument supplies the month (as a date). ' 'The second argument supplies the day of the week index as returned by extract(''dow''). ' 'The third argument supplies the zero-based index of the desired occurrence, ' 'e.g. 0 indicates the first occurrence and 1 indicates the second. A negative index will ' ' count from the end of the month, i.e., -1 is the last occurrence, -2 is the second to last ' 'occurrence. No bounds checking is done to ensure that the returned date is within the '
'specified month.';


SELECT current_date
       , nth_dow_of_month(current_date, 3, 0) as first_wed
       , nth_dow_of_month(current_date, 3, 1) as second_wed
       , nth_dow_of_month(current_date, 3, 2) as third_wed
       , nth_dow_of_month(current_date, 3, -2) as second_to_last_wed
       , nth_dow_of_month(current_date, 3, 4) as third_wed
       , nth_dow_of_month(current_date, 3, -1) as last_wed
       , nth_dow_of_month(current_date, 3, 8) as ninth_wed;
date | first_wed | second_wed | third_wed | second_to_last_wed | third_wed | last_wed | ninth_wed ------------+------------+------------+------------ +--------------------+------------+------------+------------ 2007-07-03 | 2007-07-04 | 2007-07-11 | 2007-07-18 | 2007-07-18 | 2007-08-01 | 2007-07-25 | 2007-08-29
(1 row)

Hope this helps.

Michael Glaesemann
grzm seespotcode net

[1](http://archives.postgresql.org/pgsql-sql/2007-06/msg00017.php)


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