SQLite 3.27

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create table Customer(CustomerId int, FirstName varchar(20), LastName varchar(20), Country varchar(20))
create table Invoice (CustomerId int, Total int)
insert into Customer values(1, 'A', 'A', 'A')
insert into Customer values(2, 'B', 'B', 'A')
insert into Customer values(3, 'C', 'C', 'B')
insert into Customer values(4, 'D', 'D', 'A')
insert into Invoice values(1, 100)
insert into Invoice values(1, 500)
insert into Invoice values(2, 100)
insert into Invoice values(2, 100)
insert into Invoice values(3, 100)
insert into Invoice values(4, 600)
select c.CustomerId, c.FirstName, c.LastName, c.Country, sum(Total)
from Customer c
JOIN Invoice i ON c.CustomerId = i.CustomerId
group by country, c.CustomerId
having sum(Total) in (select  max(tot) as tot2
                      from (select sum(i1.Total) as tot
                                   , country
                            FROM Customer c1
                            JOIN Invoice i1
                            ON c1.CustomerId = i1.CustomerId
                            group by i1.CustomerId) TAB
                      group by country)


CustomerId
FirstName
LastName
Country
sum(Total)


1
A
A
A
600

4
D
D
A
600

3
C
C
B
100