Joining three columns from three tables and then grouping the distinct values in the first column by the maximum value in the third column

Question

I have joined three tables into one, to receive the following table:

client_name	country_name	num_customers
Client 1	Albania	1
Client 1	Bosnia	22
Client 1	Croatia	2
Client 1	Germany	2
Client 1	England	1
Client 1	Macedonia	4
Client 1	Montenegro	3
Client 1	Serbia	4502
Client 1	Turkey	1
Client 2	Belgium	921
Client 2	England	3
Client 2	Malta	3
Client 2	Romania	7
Client 2	Sweden	1
Client 3	Latvia	1
Client 3	Lithuania	1848
Client 4	Italy	143
Client 5	Albania	1
Client 5	Algeria	4
Client 5	Armenia	4
Client 5	Austria	3
Client 5	Azerbaijan	1
Client 5	Belgium	5746

My query for joining these three tables looks like this:

SELECT c1.client_name, c2.country_name, count(c3.customer_id) AS num_customers
FROM 
clients c1
LEFT JOIN countries c2 ON c1.fk_country = c2.country_id
LEFT JOIN customers c3 ON c1.fk_customer = c3.customer_id
GROUP BY c1.client_name, c2.country_name;

NOW. I need to expand this query so that my table only includes the country per each client with the maximum number of customers: So the final table should look like this:

client_name	country_name	num_customers
Client 1	Serbia	4502
Client 2	Belgium	921
Client 3	Lithuania	1848
Client 4	Italy	143
Client 5	Belgium	5746

I tried adding a having clause or cross joining max(num_customers) but could only return one row. Help please?!

Answer 1

You can use window functions:

SELECT client_name, country_name, num_customers
FROM (SELECT c1.client_name, c2.country_name, COUNT(*) AS num_customers,
             ROW_NUMBER() OVER (PARTITION BY c1.client_name ORDER BY COUNT(*) DESC) as seqnum
      FROM clients c1 JOIN
           countries c2
           ON c1.fk_country = c2.country_id JOIN
           customers c3
           ON c1.fk_customer = c3.customer_id
      GROUP BY c1.client_name, c2.country_name
     ) ccc
WHERE seqnum = 1;

Note: I switched the joins to inner joins, under the assumptions that all clients have a country. Perhaps you have clients in Bar Tawil .

Here is a db<>fiddle showing that the syntax works.