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)
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insert into Customer values(1, 'A', 'A', 'A')
✓
insert into Customer values(2, 'B', 'B', 'A')
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insert into Customer values(3, 'C', 'C', 'B')
✓
insert into Invoice values(1, 100)
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insert into Invoice values(1, 500)
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insert into Invoice values(2, 100)
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insert into Invoice values(2, 100)
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insert into Invoice values(3, 100)
✓
select CustomerId
       , FirstName
       , LastName
       , Country
       , max(tot)
from (
select sum(i1.Total) as tot
       , i1.CustomerId
       , c1.Country
       , c1.FirstName
       , c1.LastName
FROM Customer c1
JOIN Invoice i1
ON c1.CustomerId = i1.CustomerId
group by i1.CustomerId) TAB
group by Country


CustomerId
FirstName
LastName
Country
max(tot)


1
A
A
A
600

3
C
C
B
100