Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table table1 as (
select to_date('01/01/2023','dd/mm/yyyy') date_, 30 money1 from dual union all
select to_date('10/01/2023','dd/mm/yyyy') date_, 5 money1 from dual union all
select to_date('01/02/2023','dd/mm/yyyy') date_, 10 money1 from dual union all
select to_date('10/02/2023','dd/mm/yyyy') date_, 40 money1 from dual union all
select to_date('10/04/2023','dd/mm/yyyy') date_, 40 money1 from dual
)
5 rows affected

create table table2 as (
select to_date('01/01/2023','dd/mm/yyyy') date_, 10 money2 from dual union all
select to_date('10/01/2023','dd/mm/yyyy') date_, 2 money2 from dual union all
select to_date('01/02/2023','dd/mm/yyyy') date_, 5 money2 from dual union all
select to_date('10/02/2023','dd/mm/yyyy') date_, 10 money2 from dual union all
select to_date('10/05/2023','dd/mm/yyyy') date_, 10 money2 from dual
)
5 rows affected

with t1 as(
    select sum(MONEY1) sum1, TRUNC(date_, 'MM') mon from table1
    group by TRUNC(date_, 'MM')
)
select * from t1


SUM1
MON


35
01-JAN-23

50
01-FEB-23

40
01-APR-23


with t1 as(
    select sum(MONEY1) sum1, TRUNC(date_, 'MM') mon from table1
    group by TRUNC(date_, 'MM')
),
t2 as (
    select sum(MONEY2) sum2, TRUNC(date_, 'MM') mon from table2
    group by TRUNC(date_, 'MM')
)
select coalesce(t1.mon,t2.mon), sum1, sum2, sum1-sum2
from t1
full outer join t2 on (t1.mon=t2.mon)


COALESCE(T1.MON,T2.MON)
SUM1
SUM2
SUM1-SUM2


01-JAN-23
35
12
23

01-FEB-23
50
15
35

01-MAY-23
null
10
null

01-APR-23
40
null
null

COALESCE(T1.MON,T2.MON)	SUM1	SUM2	SUM1-SUM2
01-JAN-23	35	12	23
01-FEB-23	50	15	35
01-MAY-23	null	10	null
01-APR-23	40	null	null