with tt as (
  select CustomerId, ProductId, row_number() over(partition by CustomerId order by q desc) n
    from (
      select c.CustomerID, od.ProductID, sum(od.[Quantity]) q
        from [dbo].[Customers] c
          join [dbo].[Orders] o on c.CustomerID = o.CustomerID
          join [dbo].[Order Details] od on o.OrderID = od.OrderID
        group by c.CustomerID, od.ProductID
    ) t
)
select CustomerId, ProductId
  from tt
  where n = 1;

with tt as (
  select CustomerId, CategoryID, ProductId,
        row_number() over(partition by CustomerId, CategoryId order by q desc) n
    from (
      select c.CustomerID, od.ProductID, p.CategoryID, sum(od.[Quantity]) q
        from [dbo].[Customers] c
          join [dbo].[Orders] o on c.CustomerID = o.CustomerID
          join [dbo].[Order Details] od on o.OrderID = od.OrderID
          join [dbo].[Products] p on od.ProductID = p.ProductID
        group by c.CustomerID, od.ProductID, p.CategoryID
    ) t
)
select CustomerId, CategoryId, ProductId
  from tt where n = 1;