Creating an ERD John Wright

Where to Start We need a description of some kind from interviews, questionnaires, etc Armed with the description we can apply do some english things to find the entities. There is another slide show to explain this. In this example we already have a case study document. I have highlighted candidate entities.

Local Town Library Case Study

Entities Based on this list of Candidate Entities I have chosen the following:  BOOK  PATRON  PUBLISHER  PURCHASE ORDER I did not choose the others because they don’t really play in the database we are going to build.

ERD PublisherBook Patron Purchase Order CheckOut Publishes Contains Submitted

ERD Notes Notice the many to many. These must be resolved to Associative Entities A book can have many authors and an author can publish many books, so I need to add an author entity. A Purchase order can contain many books. The question is how many. We don’t know. So like a paper version we need lines for each book. We just don’t know how many lines we need.  The answer is another table that is linked to the PO entity

ERD II Publisher Book Patron Purchase Order CheckOut Publishes Contains Submitted Author Order Item Associative Entity Writes Contains

ERD III Publisher Book Patron Purchase Order CheckOut Publishes Contains Submitted Author Order Item Associative Entity AuthorBook Contains Associative Entity