Getting number of certain days-of-the-week (weekend) from interval in PostgreSQL.

Question

Given 2 timestamps in postgres, how do you calculate the time difference without counting whole Saturdays and Sundays?

OR

How do you count the number of Saturdays and Sundays in a given time interval?

Answer 1

(days/7)*2 + number of sat/sun in the last (days%7) days.

Answer 2

This should answer the second part of your question:

create or replace function is_weekend_day(date) returns boolean
 strict immutable language 'sql'
 as $$ select case extract(dow from $1) when 0 then true when 6 then true else false end $$;

create or replace function count_weekend_days(start_date date, end_date date) returns int
 strict immutable language 'sql'
 as $$
select cast(sum(case when is_weekend_day($1 + ofs) then 1 else 0 end) as int)
from generate_series(0, $2 - $1) ofs
$$;

Making a corresponding count_non_weekend_days is simple after that.

Answer 3

The best solution to this will be to use a calendar table. This is incredibly useful and you can do all sort of interesting things - including counting the number of working days between two dates or counting the number of holidays between two dates.

This table is usually populated in advance - say for 20 years with the date for well known holidays appropriately tagged. If holidays shift, you maintain the table once in a while to mark the days as holidays. More info here and here. This uses MS SQL Server, but is easily ported as well.

Answer 4

You might find this blogpost helpful: http://www.depesz.com/index.php/2007/12/27/how-many-1sts-of-any-month-were-sundays-since-1901-01-01/

Answer 5

The following function is returning the number of full weekend days between two dates. As you need full days, you can cast the timestamps to dates before calling the function. It returns 0 in case the first date is not strictly before the second.

CREATE FUNCTION count_full_weekend_days(date, date)
  RETURNS int AS
$BODY$
  SELECT
    ($1 < $2)::int
      *
    (
      (($2 - $1) / 7) * 2
        + 
      (EXTRACT(dow FROM $1)<6 AND EXTRACT(dow FROM $2)>0 AND EXTRACT(dow FROM $1)>EXTRACT(dow FROM $2))::int * 2
        +
      (EXTRACT(dow FROM $1)=6 AND EXTRACT(dow FROM $2)>0)::int
        +
      (EXTRACT(dow FROM $2)=0 AND EXTRACT(dow FROM $1)<6)::int
    );
$BODY$
  LANGUAGE 'SQL' IMMUTABLE STRICT;

Examples:

SELECT COUNT_FULL_WEEKEND_DAYS('2009-04-10', '2009-04-20');
# returns 4

SELECT COUNT_FULL_WEEKEND_DAYS('2009-04-11', '2009-04-20');
# returns 3 (11th is Saturday, so it shouldn't be counted as full day)

SELECT COUNT_FULL_WEEKEND_DAYS('2009-04-12', '2009-04-20');
# returns 2 (12th is Sunday, so it shouldn't be counted as full day)

SELECT COUNT_FULL_WEEKEND_DAYS('2009-04-13', '2009-04-20');
# returns 2

To obtain the number of days except full weekend days, simply subtract the number of days from the function above:

SELECT
  '2009-04-20'::date
    -
  '2009-04-13'::date
    -
   COUNT_FULL_WEEKEND_DAYS('2009-04-13', '2009-04-20');