Database Entities in E/R ModelingEdit
First, an Entity is an item that exists and is discernible from other objects. An entity in terms of an E/R Model is an entity set, which is a set of entities all which are of the same type. This means that there is not just one particular occurrence of that entity, but a number of occurrences about the same thing of interest. An entity in database is a noun; that is a person, place, thing, or idea. There are different kinds of entities that an E/R Model can hold. They are traditional entities, composite entities, entities of the subtype/supertype, and strong/weak entities.
The traditional entity, also known as the simple entity, is just how it sounds it would be: The typical normal entity. This generally only has one primary key associated with it. This is never associated with many-to-many type relationships.
A composite entity is also known as a “bridge” entity. This “bridge” is used to handle the many-to-many relationships that the traditional entity could not handle. This entity lies between the two entities that are of interest and this composite entity shares the primary keys from both the connecting tables. This composite entity is also known as a “gerund” because it has the characteristics of an entity and a relationship.
A subtype/supertype is just what you would expect to see. That is the generic parent-child relationship. The supertype (parent) entity is the top most entity that shares the information down to the subtypes (children). The subtypes inherit all the information from the supertype entity. When moving down the hierarchical supertype to the subtype, that is known as specialization. When moving from the subtype to the supertype, that is known as generalization.
A weak entity is an entity that both cannot survive without the other entity that it shares a relationship with and also contains a primary key that is either partially of fully derived from the parent entity.
A strong entity is the standard database entity that has existence independence, meaning can stand alone whether another entity exists or not.