What's the difference between identifying and non-identifying relationships?

Question

I haven\'t been able to fully grasp the differences. Can you describe both concepts and use real world examples?

Answer 1

A good example comes from order processing. An order from a customer typically has an Order Number that identifies the order, some data that occurs once per order such as the order date and the Customer ID, and a series of line items. Each line item contains an item number that identifies a line item within an order, a product ordered, the quantity of that product, the price of the product, and the amount for the line item, which could be computed by multiplying the quantity by the price.

The number that identifies a line item only identifies it in the context of a single order. The first line item in every order is item number "1". The complete identity of a line item is the item number together with the order number of which it is a part.

The parent child relationship between orders and line items is therefore an identifying relationship. A closely related concept in ER modeling goes by the name "subentity", where line item is a subentity of order. Typically, a subentity has a mandatory child-parent identitying relationship to the entity that it's subordinate to.

In classical database design, the primary key of the LineItems table would be (OrderNumber, ItemNumber). Some of today's designers would give an item a separate ItemID, that serves as a primary key, and is autoincremented by the DBMS. I recommend classical design in this case.