WITH 
t3 AS 
(SELECT period FROM data d WHERE (d.id_contragent=1) AND (d.period BETWEEN '2018-10-01' AND '2018-12-01')
UNION 
SELECT period FROM data_nu d WHERE (d.id_contragent=1) AND (d.period BETWEEN '2018-10-01' AND '2018-12-01')),

t1 AS
(SELECT d.period,d.h_sum_bill,d.h_sum_change,dn.h_sum_get
FROM data d
LEFT JOIN data dn ON d.id_contragent=dn.id_contragent
    AND dn.period >= d.period + INTERVAL 1 MONTH
    AND dn.period < d.period + INTERVAL 2 MONTH 
WHERE (d.id_contragent=1) AND (d.period BETWEEN '2018-10-01' AND '2018-12-01')),

t2 AS
(SELECT d.period,d.h_sum_bill,d.h_sum_change,dn.h_sum_get
FROM data_nu d
LEFT JOIN data_nu dn ON d.id_contragent=dn.id_contragent
    AND dn.period >= d.period + INTERVAL 1 MONTH
    AND dn.period < d.period + INTERVAL 2 MONTH 
WHERE (d.id_contragent=1) AND (d.period BETWEEN '2018-10-01' AND '2018-12-01'))

SELECT t3.period,
COALESCE(t1.h_sum_bill + t2.h_sum_bill, t1.h_sum_bill, t2.h_sum_bill, 0),
COALESCE(t1.h_sum_change + t2.h_sum_change, t1.h_sum_change, t2.h_sum_change, 0),
COALESCE(t1.h_sum_get + t2.h_sum_get, t1.h_sum_get, t2.h_sum_get, 0)
FROM t3
LEFT JOIN t1 ON t1.period=t3.period 
LEFT JOIN t2 ON t2.period=t3.period