SR-ht-003 Documentation

Entities and Attributes

When writing application programs and designing databases the first step is traditionally to document program logic into what is known as flowcharts, object diagrams, and data models. Before this can be done you must first decide which technique you are going to use. The most popular form of data models is what’s known as entity-relationship model (ERD) which was designed and published for the first time ever by Peter Chan in 1976. The basic components that make up and ERD are entities, attributes, identifiers, and relationships. Entities (real world object, semantic object, physical thing, logical abstraction) are something that the user wants to keep track of and are classified and grouped as an entity class, entity classes are always capitalized. There are also different variations of entities, strong and weak. A weak entity cannot exist in a database unless another entity exists also. Anything that is not a weak entity has to be a strong entity. It is also important to follow entity integrity, an important constraint meaning that no two records are identical or duplicates (no duplicate primary keys and non-null values).
Next up is what’s known as the entity instance. First you must be able to distinguish the difference between an entity class and an entity instance. The difference between the two is that an entity class is a collection of entities described by the structure of entities in its class. There are usually many instances of an entity within the entity class. The next thing on our list is attributes. Attributes (describe the entities) are what describes an entities characteristic, and are printed in a combination of lower case and uppercase letters. It is normal to assume that the instances of an entity class have all the same attributes. An attribute consists of a data type ranging from, but not