Я не согласен с вами. По крайней мере, в данных образца, он работает правильно.
Для 12 октября 2019 года
SQL> with L_DEA (PaySum, PlanDate) as
2 (select 23928.38 , to_date('14.10.2019','dd.mm.yyyy') from dual union all
3 select 24347.13 , to_date('12.11.2019','dd.mm.yyyy') from dual union all
4 select 24773.20 , to_date('12.12.2020','dd.mm.yyyy') from dual union all
5 select 24773.20 , to_date('10.02.2020','dd.mm.yyyy') from dual union all -- added this
6 select 25206.73 , to_date('13.01.2020','dd.mm.yyyy') from dual )
7 select * --sum(s.PaySum)
8 from L_DEA s
9 where s.PlanDate between trunc(date '2019-10-12' + 1) and
10 ADD_months(trunc(date '2019-10-12' + 1), 3)
11 --and ID = :iId;
PAYSUM PLANDATE
---------- ---------
23928.38 14-OCT-19
24347.13 12-NOV-19
25206.73 13-JAN-20
SQL>
для другой даты, т.е. 13 октября 2019 года
SQL> with L_DEA (PaySum, PlanDate) as
2 (select 23928.38 , to_date('14.10.2019','dd.mm.yyyy') from dual union all
3 select 24347.13 , to_date('12.11.2019','dd.mm.yyyy') from dual union all
4 select 24773.20 , to_date('12.12.2020','dd.mm.yyyy') from dual union all
5 select 24773.20 , to_date('10.02.2020','dd.mm.yyyy') from dual union all -- added this
6 select 25206.73 , to_date('13.01.2020','dd.mm.yyyy') from dual )
7 select * --sum(s.PaySum)
8 from L_DEA s
9 where s.PlanDate between trunc(date '2019-10-13' + 1) and
10 ADD_months(trunc(date '2019-10-13' + 1), 3)
11 --and ID = :iId;
PAYSUM PLANDATE
---------- ---------
23928.38 14-OCT-19
24347.13 12-NOV-19
25206.73 13-JAN-20
SQL>
Ура !!