Entity Relationship Modeling
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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.
In the analysis phase, we are concerned about finding out about the business. At this early stage, we normally do not talk about performance-specific issues. The main goal of the Analysis phase is to determine what information the business needs to perform a certain task (accounting, invoicing, customer support), how that information should be best possibly organized, and what are the relationships between the different sets of information. We start with a few definitions:
Entity-An entity is a specific object of interest to the business area. We might say that in an accounting system, two things are basically necessary: Accounts and transactions. In a customer support system, we would need to have some information about customers, for a start. Each such unit of essential information is named ‘an entity’, with a name and attributes.
Attributes-Each entity will normally have one or more attributes. Attributes may be thought of as smaller pieces of information within an entity. Together they describe our entity to the degree we find necessary. An example is a customer entity:
The entity is named CUSTOMER, and has two attributes, that we have found to be important to know about the customer: Customer number and customer name. Most likely, the customer will have several other attributes as well, but that will show up during the analysis phase. The attributes have a # and a * in front of them: A # means that the attribute is (part of) the primary key for that entity. The primary key is a unique value for every new customer we get so that we can uniquely identify him from all the other customers. The attribute NAME has a * in front: This indicates that it is a mandatory attribute; for every new customer, name MUST be filled in. The opposite is also possible; if an attribute has an o in front of it, it is an optional attribute, and a value is not required.
Relationships-In a relational database, all entities have bonds between them, expressed as relationships. A relationship is a link between to entities, and it tells us something about which relationships exists between our entities. The following example illustrates this:
This sketch tells us that a given account MAY have zero or more balances, but a given balance MUST BE valid for one and only one account. That sounds reasonable. Reading ER diagrams with relationships, gives us a very quick view of the logical structure of a database in the making, without thinking about the physical implementation of it, at this point.
These are the basic elements you need to know about before starting out on your ER modeling: Entities, their attributes, what uniquely identify an occurrence of an entity, and what the relationships between the different entities are.
Next: The Entity >>
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