Equivalent to WORKDAY function in Excel/Google Sheet (does not support holidays, only handles weekends as saturday/sunday)

f_add_business_day.sql

CREATE OR REPLACE FUNCTION f_add_business_day(start_date date, num_days integer)
  RETURNS date AS
$BODY$
SELECT COALESCE(
    (
        SELECT workdays.date
        FROM (
            SELECT calendar.date::date,
                   row_number() OVER (ORDER BY CASE WHEN num_days = abs(num_days) THEN calendar.date END, calendar.date DESC) as elapsed_days
            FROM generate_series(
                /* start the calendar from day +1 or -1 depending on the sign of num_days */
                start_date + (CASE WHEN num_days = abs(num_days) THEN 1 ELSE -1 END),
                /* until far enough in the future (or past). It is 2*num + 5 days away to cover enough range */
                start_date + (((abs(num_days) * 2) + 5) * (CASE WHEN num_days = abs(num_days) THEN 1 ELSE -1 END)),
                /* increment/decrement by 1 day */
                '1 day'::interval * (CASE WHEN num_days = abs(num_days) THEN 1 ELSE -1 END)
            ) calendar(date)
            /** remove the weekends if there were any holidays we could hardcode them here */
            WHERE EXTRACT('dow' FROM calendar.date) NOT IN (0, 6)
        ) workdays
        /* find the right day N days away */
        WHERE elapsed_days = abs(num_days)
    ),
    /* handle the case when the input is 0, it would not match any row, so return same date */
start_date)
$BODY$
  LANGUAGE sql IMMUTABLE
  COST 100;