data-modeling

Our application has an online shop among other features, and users are normally requested to register before completing a sale, creating a unique customer_ID in the process. When they return, they can log in and their contact details and transaction history are retrieved from the database. We are now exploring what to do in the case of an 'anonymous' or 'guest' customer, opening up the online shop to customers who don't want to register, and also for sales logged in the backend application, where taking the customer's email, postal address, etc is just too time consuming. The solution has

I know a lot of answers have been given that advise embedding an array of references to other users, but what all answers of this sort neglect is that friendship is a two-way relationship and that when Alice is Bob's friend, Bob is automatically Alice's friend. I'm not modeling followers. So I don't want to keep two references every time a new user enters the system. I need a model that takes into account the two-way nature of the relationship. I was thinking a collection of Friendship 'edges' where each document holds a reference to the two users. Wanted to know if literature regarding this

I have created a Database in DB2 and tables with relationships. I would like to create a ER diagram based on my database design in DB2. MS SQL has a facility to create ER diagrams from DB schema, but DB2 doesn't seem to have one, at least to my knowledge. Any one know of any open source tools/facility within DB2 itself for this? You could try TOAD for DB2 (freeware and commercial versions). Download here Also IBM Data Studio looks promising. DbVisualizer can visualize (and much more) just about any database, provided that the relevant foreign keys have been defined. DbVisualizer isn't open

I would like to know if there is a standard or generally accepted way of representing an equivalent of NULL used in databases for RDF data. More specifically, I'm interested in a way to distinguish the following cases for a value o of a property p ( p is the predicate, o the object of an RDF triple): The value is not applicable , i.e. property p does not exist or does not make sense in the context. The value is unknown , i.e. it should be there but we don't know it. The value doesn't exist , i.e. the property doesn't have a value (e.g. year of death for a person alive). The value is witheld ,