SQL find the maximum value - sql

Question

I have 2 tables:
Customer table
CustomerId
FirstName
LastName
Country
1
Luís
Gonçalves
Brazil
2
Leonie
Köhler
Germany
3
François
Tremblay
Canada
4
Bjørn
Hansen
Norway
52
Emma
Jones
United Kingdom
53
Phil
Hughes
United Kingdom
Invoice table
InvoiceId
CustomerId
Total
1
2
1.98
2
4
3.96
3
8
5.94
140
52
23.76
369
52
13.86
283
53
28.71
109
53
8.91
I have to write a query that returns the country along with the top customer and how much they spent. For countries where the top amount spent is shared, provide all customers who spent this amount.
I wrote a query like:
SELECT c.CustomerId, c.FirstName, c.LastName, c.Country, SUM(i.Total) AS TotalSpent
FROM Customer c
JOIN Invoice i
ON c.CustomerId = i.CustomerId
GROUP BY c.FirstName, c.LastName
HAVING i.Total >= MAX(i.Total)
ORDER BY c.Country;
the query is not finding the maximum values but it is returning all available values.
I am not sure about which DBMS is used as it is my first steps in SQL coding and above example is from Udacity learning platform lab (Maybe it is SQLite that they are using in the lab)
Any help is appreciated. Thank you!

Answer 1

You did not shared your database with us.
Also, you need to add expected results in your question from the data you provided.
But lets say you use SQLite then this would work I think:
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
DEMO
After the comment from the OP I have edited the code:
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)
DEMO2

Answer 2

As the accepted (at the time of writing this answer) solution would fail on at least PostgreSQL (for not including a selected value in either the group by clause or an aggregate function) I provide another variant:
WITH t AS
(
SELECT
c.customerid AS i, c.last_name AS l, c.first_name AS f,
c.country AS c, SUM(i.total) AS s
FROM customer c JOIN invoice i ON c.customerid = i.customerid
GROUP BY c.customerid, c.country
-- note: last and first name do not need to be included in the group by clause
-- here as the (assumed!) primary key 'customerid' already is
)
SELECT c, l, f, s FROM t
WHERE s = (SELECT MAX(s) FROM t WHERE t.c = c)
(tested on PostgreSQL)

Answer 3

Below code worked fine to fulfill all the requirements:
WITH tab1 AS
( SELECT c.CustomerId, c.FirstName, c.LastName,c.Country, SUM(i.Total) as TotalSpent
FROM Customer AS c
JOIN Invoice i ON c.CustomerId = i.CustomerId
GROUP BY C.CustomerId
)
SELECT tab1.*
FROM tab1
JOIN
( SELECT CustomerId, FirstName, LastName, Country, MAX(TotalSpent) AS TotalSpent
FROM tab1
GROUP BY Country
)tab2
ON tab1.Country = tab2.Country
WHERE tab1.TotalSpent = tab2.TotalSpent
ORDER BY Country;