Top Group BY Problem DB2 [duplicate] - group-by

Question

I've been trying for hours but can't get the query to do what I want using DB2.
From table Company and Users I have the following tickets quantity info per company/user
user company quantity
------------ ------------ ------------
mark nissan 300
tom toyota 50
steve krysler 80
mark ford 20
tom toyota 120
jose toyota 230
tom nissan 145
steve toyota 10
jose krysler 35
steve ford 100
This is generated by the query:
SELECT T.USER, COUNT(T.USER) AS QUANTITY, T.COMPANY FROM TICKET T
INNER JOIN COMPANY P ON P.COMPANY = T.COMPANY
GROUP BY (T.USER, T.COMPANY) -- ORDER BY QUANTITY DESC
What I want to see is the top user for each company, so given the data above, the query should show me:
user company quantity (Top user per company)
------------ ------------ --------------------------------
mark nissan 300
jose toyota 230
steve ford 100
steve krysler 80
How can I write the SQL to return this result?
Final answer (noted in a comment):
SELECT user, quantity, company
FROM (SELECT user, quantity, company,
RANK () OVER (PARTITION BY company ORDER BY quantity DESC) AS r
FROM (SELECT T.USER, COUNT(T.USER) AS QUANTITY, T.COMPANY
FROM TICKET T JOIN COMPANY P ON P.COMPANY = T.COMPANY
GROUP BY (T.USER, T.COMPANY) ) s ) t
WHERE r = 1;

Answer 1

Build it up step by step.
Find the maximum quantity for each company, assuming the first data table shown in the question is called 'Tickets':
SELECT Company, MAX(Quantity) AS MaxQuantity
FROM Tickets
GROUP BY Company;
Now, find the data for the user(s) with that maximum quantity for that company:
SELECT T.User, T.Company, M.MaxQuantity
FROM Tickets AS T
JOIN (SELECT Company, MAX(Quantity) AS MaxQuantity
FROM Tickets
GROUP BY Company) AS M
ON T.Company = M.Company AND T.Quantity = M.MaxQuantity;
If the top quantity for a particular company was, say, 200 and two users both scored 200 for that company, then this query lists both users.
Now, if you mean that the query you show in the question generates the first result table, then what I called tickets just above needs to be the derived table:
SELECT T.User, COUNT(T.User) AS Quantity, T.Ccompany
FROM Ticket AS T
INNER JOIN Company AS P ON P.Company = T.Company
GROUP BY (T.User, T.Company)
ORDER BY QUANTITY DESC
In which case, we can use a WITH clause (syntax unchecked, but I think it is correct per SQL standard):
WITH Tickets AS
(SELECT T.User, COUNT(T.User) AS Quantity, T.Ccompany
FROM Ticket AS T
JOIN Company AS P ON P.Company = T.Company
GROUP BY (T.User, T.Company)
)
SELECT T.User, T.Company, M.MaxQuantity
FROM Tickets AS T
JOIN (SELECT Company, MAX(Quantity) AS MaxQuantity
FROM Tickets
GROUP BY Company) AS M
ON T.Company = M.Company AND T.Quantity = M.MaxQuantity;
Clearly, you can also write the WITH sub-query out twice if you prefer.

Answer 2

This should work. Create a derived view to calculate the Quantity per user and per company. Then get the max of then Quantity and then join the max back to the the calculation of the quantity.
SELECT p.company,
t.user,
t.quantity
FROM (SELECT MAX(t.quantity) max_quantity,
t.company
FROM (SELECT
COUNT(t.user) quantity,
t.company
FROM ticket t
GROUP BY t.company) t) maxq
INNER JOIN (SELECT t.user,
t.company,
COUNT(t.user) quantity
FROM ticket t
GROUP BY t.company,
t.user) t
ON maxq.max_quantity = t.quantity
AND maxq.company = t.company
INNER JOIN company p
ON p.company = t.company
ORDER BY t.quantity DESC
A working sample that shows the top users by tag for the StackOverflow data can be found here.