pobieranie danych dla jednego rekordu z innego wczesniejszego rekordu

SELECT
    sum(saldo) AS saldo,
    extract(doy from valid_from_date) AS day_of_year,
    extract(week from valid_from_date) AS week_of_year,
    extract(month from valid_from_date) AS month_of_year,
    extract(quarter from valid_from_date) AS quarter,
    extract(year from valid_from_date) AS year,    
    count(id) AS count
FROM
    balance
WHERE
    (valid_from_date BETWEEN '2011-01-01' AND '2011-01-31') AND
    extract(dow from valid_from_date) NOT IN (0, 6) AND -- vide dokumentacja
    ghost = false AND
    product_3_id IN (59, 58, 60) AND
    product_group_id IN (3)
GROUP BY
    valid_from_date
ORDER BY
    day_of_year;

CREATE OR REPLACE FUNCTION getNextBusinessDay(date)
RETURNS date AS $$
    SELECT
        CASE date_part('isodow', $1)
            WHEN 5 THEN CAST ($1 + interval '3 days' AS date) -- piątek
            WHEN 6 THEN CAST ($1 + interval '2 days' AS date) -- sobota
            ELSE CAST ($1 + interval '1 days' AS date) -- pozostałe
        END;
$$ LANGUAGE SQL;

/* Zwraca prawdę jeżeli dzień jest wolny od pracy,
   a fałsz w przeciwnym wypadku */
CREATE OR REPLACE FUNCTION isdayfree(date)
RETURNS boolean AS $BODY$
BEGIN   
    IF EXISTS (SELECT free_day FROM days_free WHERE free_day = $1) -- zakładając, że free_day ma typ date
        OR date_part('isodow', $1) IN (6, 7)
    THEN
        RETURN true;
    ELSE
        RETURN False;
    END IF;
END;
$BODY$
LANGUAGE plpgsql;

/* Zwraca następny dzień biznesowy */
CREATE OR REPLACE FUNCTION getNextBusinessDay(date)
RETURNS date AS $BODY$
DECLARE
    resultDate date := $1;
BEGIN
    resultDate := resultDate + 1;
    WHILE isdayfree(resultDate) = true LOOP
        resultDate := resultDate + 1;
    END LOOP;
    RETURN resultDate;
END;
$BODY$
LANGUAGE plpgsql;

SELECT
    sum(saldo) AS saldo,
    date_part('doy', next_day) AS day_of_year,
    date_part('week', next_day) AS week_of_year,
    date_part('month', next_day) AS month_of_year,
    date_part('quarter', next_day) AS quarter,
    date_part('year', next_day) AS year,    
    count(id) AS count
FROM 
    balance a INNER JOIN (SELECT id, getNextBusinessDay(valid_from_date) AS next_day FROM balance) b
    USING (id)
WHERE
    (next_day BETWEEN '2011-01-01' AND '2011-01-31') AND
    isdayfree(valid_from_date) = false AND
    ghost = false AND
    product_3_id IN (59, 58, 60) AND
    product_group_id IN (3)
GROUP BY
    next_day
ORDER BY
    day_of_year;

 saldo | day_of_year | week_of_year | month_of_year | quarter | year | count
-------+-------------+--------------+---------------+---------+------+-------
  2600 |           4 |            1 |             1 |       1 | 2011 |     3
   400 |          10 |            2 |             1 |       1 | 2011 |     1
(2 rows)