SELECT nb.service_id, (nb.summ  -  ac.summ), nb.summ, ac.summ, nb.service_id, ac.service_id
FROM n_accountservice ass
JOIN n_balances nb ON nb.account_id = ass.account_id and nb.service_id = ass.service_id
JOIN n_accruals ac ON ac.account_id = ass.account_id AND ac.service_id = ass.service_id
WHERE --ass.state_srv = 1 AND
	 ass.account_id = '000000204909'
	AND nb.DATA = add_months(trunc(to_date('01.08.2017','dd.mm.yyyy'), 'mm'), -2)
	AND ac.DATA = add_months(trunc(to_date('01.08.2017','dd.mm.yyyy'), 'mm'), -1)
--	AND nb.summ  <=  ac.summ
;
27	0	0	0	27	27