WITH
t3 AS 
(SELECT '2020-04-01' period),

t1 AS
(SELECT period, h_sum_debt, hw_sum_debt
FROM data_sgrc
WHERE (id_contragent=1) AND (period='2020-04-01')),

t2 AS
(SELECT period, h_sum_debt, hw_sum_debt
FROM data_sgrc_nu
WHERE (id_contragent=1) AND (period='2020-04-01'))

SELECT 
	COALESCE(t1.h_sum_debt + t2.h_sum_debt, t1.h_sum_debt, t2.h_sum_debt, 0) hsd,
	COALESCE(t1.hw_sum_debt + t2.hw_sum_debt, t1.hw_sum_debt, t2.hw_sum_debt, 0) hwsd
FROM t3
LEFT JOIN t1 ON t1.period=t3.period 
LEFT JOIN t2 ON t2.period=t3.period