Get dates of a day of week in a date range

Question

I need a function in PostgreSQL that accepts a date range and returns the dates inside the date range that are Mondays. Anybody have an idea how this could be done?

Answer 1

create function f(dr daterange)
returns setof date as $$
    select d::date
    from generate_series(
        lower(dr), upper(dr), interval '1 day'
    ) s (d)
    where
        extract(dow from d) = 1 and
        d::date <@ dr;
    ;
$$ language sql;

select f(daterange('2014-01-01', '2014-01-20'));
     f      
------------
 2014-01-06
 2014-01-13

Answer 2

The most efficient way should be to find the first Monday and generate a series in steps of 7 days:

CREATE OR REPLACE FUNCTION f_mondays(dr daterange)
  RETURNS TABLE (day date) AS
$func$
SELECT generate_series(a + (8 - EXTRACT(ISODOW FROM a)::int) % 7
                     , z
                     , interval '7 days')::date
FROM  (
   SELECT CASE WHEN lower_inc(dr) THEN lower(dr) ELSE lower(dr) + 1 END AS a
        , CASE WHEN upper_inc(dr) THEN upper(dr) ELSE upper(dr) - 1 END AS z
   ) sub
$func$ LANGUAGE sql;

• The subquery extracts start (a) and end (z) of the range, adjusted for inclusive and exclusive bounds with range functions.

• The expression (8 - EXTRACT(ISODOW FROM a)::int) % 7 returns the number of days until the next monday. 0 if it's Monday already. The manual about EXTRACT().

• generate_series() can iterate any given interval - 7 days in this case. The result is a timestamp, so we cast to date.

• Only generates Mondays in the range, no WHERE clause needed.

Call:

SELECT day FROM f_mondays('[2014-04-14,2014-05-02)'::daterange);

Returns:

day
----------
2014-04-14
2014-04-21
2014-04-28

SQL Fiddle.