An entity represents a collection of information about something that the business deems important and worthy of capture. A noun identifies a specific entity. Usually an entity fits into one of several categories - who, what, when, where, why, or how.

Categories of an Entity


Who is important to the business?
Example: Employee, Customer, Vendor, Competitor.


What is important to the business?
Example: Product, Service, Raw Material, Finished Good.


When is the business in operation?
Example: Time, Date, Month, Fiscal Period.


Where is business conducted?
Example: Mailing Address, Distribution Point, Website URL, IP address.


Why is the business in business?
Example: Order, Return, Complaint, Withdrawal, Deposit, Inquiry, Trade, Claim.


How does the business stay in business?
Example: Invoice, Contract, Agreement, Account, Purchase Order.

Entity Instance 

Entity instances are the occurrences or values of a particular entity. For example, the entity Student may have multiple instances with names Raj, Rahul, Kumar and so forth.

Entity at different data modeling levels

In the previous post we discussed about the three levels of data modelling. Entities are components of all the three levels, such as conceptual, logical, physical.

In conceptual level, the entities are both basic and critical to the business. At universal level, there are certain subject areas common to all companies, such as Customer, Product and Employee. Making scope slightly narrower, a given industry may have certain unique subject areas. For example, Drug Name may be a valid subject area for Medicine Company, but perhaps not for other industries.

In logical level, entities represents the business at a more detailed level than at the subject area level. In general, a subject area entity represents many logical model entities. For example, entity Product in Conceptual level, could produce a large number of logical entities, including Goods and Service.

Entities in physical level correspond to database tables. One logical entity can break down into several physical tables, and even more frequently, one physical table can be created from many logical entities. For example, Customer Name entity in logical level can be break down into Customer First Name, Middle Name, Last Name and entities in logical level such as Email Address and Web Address can be combined into the same physical entity.

Types of Entities

There are two types of entities, such as Strong entity and Weak entity. Usually, Strong entities are represented by a rectangle with straight right angle corners and Weak entities are represented by a rectangle with rounded corners.

Strong entities are stand on their own. They represents one occurrence of a person, place or thing independent of any other entities. In order to find the information about a particular Employee, for example its Employee Identifier could be used to retrieve it from the database. 

Weak entities need to rely on at least one other entity. This means you cannot retrieve an entity instance without referring to an entity instance from another entity. For example, Present Age can be retrieved by a Date of birth, in combination with something within the Present Age as a Sequence Number.

We know that, a data model is a communication tool. Representing the strong and weak entities in different diagrams on the model help us understand the relationships and dependencies between entities. For example, an application developer, working on a recruitment software, will understand that, Present Age is representing in weak entity is dependent on the Date of Birth, need to get the Date of Birth before calculating age requirement from present age.

I hope this post will give you basic understanding about entities. In the coming post let us discuss about the data fields and domain.

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