My guess would be add a store table with the store information.
Then add a store column as a foreign key reference to the store table in the Product table (so you can manage inventory quantities in separate stores) and a store column as a foreign key reference to the store table int he Sale table (so you can link a sale to a specific store)
The store in the product table would become part of the primary key along with product ID.
I would suggest normalizing that a bit more though and splitting the product table into two tables - product and product_store, where product contains the general information about the product (name, description, category etc etc) and product_store contains the store specific information against the product (product id, store id, quantity in stock, quantity on back-order with suppliers etc) - In this case the product_store table would have the store key and not the product table.
That was my first approach except with the Categories table. However, that doesn't work because several of the other tables are also related to the store (Such as Sales and taxes etc.)
I could at a Store_ID column to those tables too. Unfortunately, then it would be possible to have a category/product in one store linked to a tax in a different store! That can't be right.
My guess is it has something to do with adding a store_id column to every table and making that part of the table key. I just don't know enough about database design to know for sure or exactly how to do that.
My approach generally goes by "understanding" the key event(s) of the system. In this case i guess is the sale.
So i would ask "where" the sale occurs? the answer is: in the store. So establish a relation between sales and stores.
Also ask "what" is sold? The answer: a product, so i would associate sales with products not with categories (nor category_sale).
Also talking about connectivity/cardinality if the head of the arrow means the "one" side then a lot of relations seem (very) wrong to me (or vicevers)
It all depends on what you need. If you are writing a database to be used by 1 big company, that has multiple stores, it is the easiest. All products then belong to the same category, have the same image, tax, discount category, ...
If you plan to make 1 data model to serve completely unrelated shops with their own product-images, discount-categories, tax, ... you will have to make a more profound remodelling.
Unfortunately, then it would be possible to have a category/product in one store linked to a tax in a different store! ...
My guess is it has something to do with adding a store_id column to every table and making that part of the table key.
Good thinking! It is a way to let the database protect its integrity.
But in practice I would not implement it that way. It has serious drawbacks when you have to maintain your database. A PK can become quite long as it may consist of many FK's. Just go on as you already did in your model, give each table an ID as PK and let the FK's be no part of the PK.
I agree with the remarks of Carloco.
With kind regards . . . . . SQL Server 2000/2005/2012
Grabel's Law: 2 is not equal to 3 -- not even for very large values of 2. Pat Phelan's Law: 2 very definitely CAN equal 3 -- in at least two programming languages