Stuck doing a common table expression to calculate total orders for each product category using NORTHWND - sql

Question

Guys so as part of my job as a Data Support Analyst I am training up to become a software developer, my mentor gave me a group of test statements and this one seems way more advanced than anything I have done previously. The question is...
*
6) Write a subquery or common table expression to calculate total
orders for each product category. that will Write a query that will
bring back each product name, the total number of orders made for the
product and join to the subquery or CTE to calculate the percentage
of sales that product represents of its product category. For example,
if Category of Soaps has 2 products; Blue Soup and Red Soap, blue soup
had 40 orders and red soaps had 10 orders I would expect to see the
following rows: Product Name Total Orders % of Category Red
Soap 40 80%
Blue Soap 10 20%
*
So far I have managed to get the following but I'm struggling to progress past this...
;WITH [Products] (CategoryId, TotalNumberOfOrders)
AS (
SELECT p.CategoryId,
COUNT (OD.OrderID) as TotalNumberOfOrders
FROM [dbo].[Products] p
INNER JOIN [dbo].[Order Details] OD
ON p.ProductID=OD.ProductID
GROUP BY p.CategoryId )
SELECT * From Products
Any help would be fantastic! ( I'm using NORTHWND database btw)

Answer 1

I think what you need is
;WITH temp AS
(
SELECT p.CategoryId,
COUNT (DISTINCT OD.OrderID) as TotalNumberOfOrders
FROM [dbo].[Products] p
INNER JOIN [dbo].[Order Details] OD ON p.ProductID=OD.ProductID
GROUP BY p.CategoryId
)
SELECT p.ProductName,
Count(DISTINCT Od.OrderId) AS Total,
Count(DISTINCT Od.OrderId)*100/temp.TotalNumberOfOrders AS Percentage
FROM Products p
INNER JOIN [dbo].[Order Details] OD ON p.ProductId = OD.ProductId
INNER JOIN temp ON p.CategoryId = temp.CategoryId
GROUP BY p.ProductId, p.ProductName, temp.TotalNumberOfOrders
Remember Count(Distinct..) to count Order.

Answer 2

Using a sub-Query:
SELECT P.ProductName,
COUNT(OrderID) AS NumberOfOrders,
CAST((CAST(COUNT(OrderID) AS DECIMAL(9,2)) / CAST(Derived_Table.CNT AS
DECIMAL(9,2))) * 100 AS DECIMAL(9,2)) AS Percentage
FROM [Northwind].[dbo].[Order Details]
INNER JOIN dbo.Products AS P
ON [Order Details].ProductID = P.ProductID
INNER JOIN (SELECT COUNT(F.OrderID) AS CNT, P.CategoryID
FROM [Northwind].[dbo].[Order Details] F
INNER JOIN dbo.Products P
ON F.ProductID = P.ProductID
GROUP BY P.CategoryID) AS Derived_Table
ON P.CategoryID = Derived_Table.CategoryID
GROUP BY [Order Details].ProductID, P.ProductName, Derived_Table.CNT
ORDER BY [Order Details].ProductID