Entity Relationship Modeling (ER modeling) is by far the most common way to express the analytical result of an early stage in the construction of a new database. In this ebook, Alf Pedersen describes the principles for ER modeling, as well as the most important terms used in modeling a new database.
Entity Relationship Modeling - The Entity (Page 2 of 21 )
The single entity-As we define different entities, we find that we are digging deeper and deeper into the problem domain of the business we are modeling. The more we, as system analysts, interact with the business we are modeling with, the more information we find belong in different entities. This is how the process works.
You should not be afraid to create new entities, if so just as placeholders for ideas. In the analysis phase we need not be concerned about it; the analysis phase is where we test out ideas, new questions, and eventually evolve towards a solution which is sound, correct, and shows the business as it really is (or as we would like it to be).
It is also a good idea to create attributes as they are coming up. Again, they can be evaluated and thrown away later if we really do not need them. With a good ER modeling tool, your ER diagram may be thought of as a sketchbook, especially in an early phase of analysis.
Single entities are relatively easy to spot: An account is something different from transactions or balances, and a customer is something different from an order. However, customers and orders are related to each other through relationships, as well as accounts are related to transactions and balances. Sometimes we may enter a dilemma, when two or more entities actually look very similar: What is really the difference between a customer and a supplier?
Both entities have some form of identification; they both have a name, address, telephones, contact persons etc. We may even have a discount system for our customers, and our suppliers also give us some discount mechanisms. The two are really quite like.
In our analysis work, we have two mechanisms for dealing with such uncertainties.
The super-entity and the sub-entity-If we first draw an entity we call BUSINESS CONTACT, and then draw two or more entities inside the BUSINESS CONTACT entity, then BUSINESS CONTACT becomes a super-entity, and the others become sub-entities of BUSINESS CONTACT. The purpose of this is to verify if we really are talking about different kinds of contacts:
As we add attributes into this model, we must ask ourselves: The attribute I am about to create; is it valid for all the sub-entities, or is it specific to only one (or two...) of the sub-entities? If the attribute is valid for all sub-entities, we place it in the super-entity in stead.