Primary key advice for simple table I'm making a sample database to test myself in MySQL workbench and was wondering the following:

I have five tables, Customer, Product, Stock(Quantity), Warehouse and Purchase Order.

For primary keys I have customerId, productId, warehouseId, and invoiceNo...but what would the stocks primary key be? Apologies if the answer is obvious, but as I understand it the foreign key would be the productNo.

Very new to this!

Any input would be greatly appreciated!
Given with STOCK the same product could be multiple warehouses, the combined (productCode, warehouseCode) is the unique element for this table and that should be the primary key.