Question

I am developing a stock ordering system in PHP/MySQL. Products can have multiple suppliers. Products are added to the basket by specifying the quantity required and the supplier. This is the current DB Structure:

Products
=========
id (PK)
name
etc...

Suppliers
==========
id (PK)
name
etc...

Product_Suppliers
=================
id (PK)
product_id (PK)
supplier_id (PK)
price

Basket
=======
product_id (PK)
quantity
product_suppliers_id

The Product_Suppliers table stores the different suppliers and their prices for each product.

When adding a product to the basket, it first checks to see if the product_id already exists in the Basket table – if it does then it will simply overwrite the record. If it does not exist then it will create a new record.

The product_suppliers_id field in the Basket table specifies which supplier has been selected for that particular product.

I wanted to know whether I have got the correct approach here, or whether I need to make any changes to eliminate any redundancy.

Answer 1

In Basket, you should have:

basket_id (PK)
product_id
supplier_id
quantity
price

Product_Suppliers is merely a configuration table. It has the only the current configuration of a product offered by a Supplier. Your Basket should ‘know better’ then pointing to Product_Suppliers because:

• Products can change price.
• Products can also cease to be offered by a Supplier at a given point.

Your design should allow you to query for historical data correctly, even if a product changes the price or if it’s not offered by the supplier anymore.

Thus, you should not have product_suppliers_id in it.

Also, I don’t see in your design whose basket that is, meaning you should also have a table for Clients or Users (or both, if users can input their own orders AND the store internal users can input orders for the clients) and the corresponding key(s) (client_id, user_id).