SQL comparison report on cartesian product using subquery - sql

Question

I'm a student building a comparison report query in MySQL on a database that tracks customers, products, and purchases in separate tables. I have to create a report that shows how many products were sold every month for each province using a subquery. I was told to use a cross join between product and customer, however, my query runs into a problem when I try to group them as the records all collapse into each other and I don't understand why this is happening. I'm not sure if this is the correct way to approach this problem since my customer and product table don't have any values that intersect with each other except through the purchase table.
These are my create table scripts
CREATE TABLE 'customer' (
'CustomerID' INT NOT NULL,
'City' VARCHAR(100) NOT NULL,
'Province' CHAR(2) NOT NULL,
PRIMARY KEY ('CustomerID'));
CREATE TABLE 'product' (
'ProductID' INT NOT NULL,
'ProductName' VARCHAR(100) NOT NULL,
'Price' DECIMAL(5,2) NOT NULL,
PRIMARY KEY ('ProductID'));
CREATE TABLE 'purchase' (
'PurchaseID' INT NOT NULL,
'PurchaseDate' DATE NOT NULL,
'customer_CustomerID' INT NOT NULL,
'product_ProductID' INT NOT NULL,
PRIMARY KEY ('PurchaseID'),
CONSTRAINT 'fk_purchase_customer'
FOREIGN KEY ('customer_CustomerID')
REFERENCES 'customer' ('CustomerID'),
CONSTRAINT 'fk_purchase_product'
FOREIGN KEY ('product_ProductID')
REFERENCES 'product' ('ProductID'));
This is the query that I have written as I have understood the instructions.
SELECT DISTINCT province, productName AS Product, JanTotalSales
FROM PRODUCT cross join CUSTOMER
LEFT JOIN
(
SELECT purchaseID, product_productID, customer_customerID, COUNT(purchaseDate) AS JanTotalSales
FROM PURCHASE
WHERE MONTH(purchaseDate) = 01
)JAN ON PRODUCT.productID = JAN.product_productID
GROUP BY province, productID;
I should be getting results like this
Province
Product
JanTotalSales
FebTotalSales
...
TotalSales
QC
Paper
1
NULL
...
1
ON
Paper
1
2
...
3
AB
Paper
1
NULL
...
1
AB
Wire
2
2
...
4
ON
Wire
2
1
...
3
NULL
Kit
NULL
NULL
...
NULL
SK
Gummy
1
1
...
2
NULL
Bag
NULL
NULL
...
NULL
However, I receive results like this when I do it on the January subquery.
Province
Product
JanTotalSales
AB
Paper
NULL
AB
Wire
NULL
AB
Kit
NULL
AB
Kit
13
ON
Paper
NULL
ON
Wire
NULL
ON
Kit
NULL
ON
Kit
13
I appreciate whatever help you can give to show me where I'm going wrong. From what I understand it's something to do with how the grouping occurs but I can't figure out why.