Entities

An entity is an object with a distinct group of properties which is simply identified. Entities are the building blocks of a database.  Some instances of entities are Teacher, Student and Classroom. You represent an entity using a rectangular box which contains the name of the entity.

An entity instance is a specific value of an entity. The Entities are the basic building blocks of relational database design. An entity describes any place, person, thing or concept for that data will be collected. Some instance of entities involving the following: The highlighted words or phrases are likely candidates for modeling as an entity. The teacher or professor,  concept  is  an  clear  choice  as  are  the  ideas  of  a  course  and department. The DB should have to keep track of several different teachers and courses. Further, the database will be used over several semesters, so if detailed information required to be kept about each semester, the thought of a semester   should be modeled as an entity. Notice that the idea of a campus, although important to a college and is not a candidate for an entity in this instance. This is because a small college with only one campus will possibly build any campus-specific information directly into its data model. If the college were setting up to expand to multiple campuses in the future and creating a campus entity then would be important because it would permit you to gather information about each distinct campus.

A good practice is to list all the entities in your database with a one sentence description of what which entity represents. For instance, a teacher should be defined as a person employed through the college who is responsible for instructing students in a class. generally; a good entity can be described in one sentence otherwise it is a very abstract concept.

Review the case study to see if you can recognize some entities which will be used in the data model. Then review the following list of entities that are given below:

- Teacher: A person employed through the college who is responsible for instructing a class sessions.

- Student: Student is a person who is enrolled in classes at the college and attends class sessions.

- Course:  The learning material taught in a class

- Class: A scheduled example of a course which is taught through one teacher and that meets in a particular room during specific times of the week.

- Class Session: An example of a class which occurs at a particular time and date.

When speaking of an entity in the data model one commonly is referring to an example of that entity. An example is a particular occurrence of an entity which is obvious from all other occurrences of that entity. For instance, the college has multiple professors, like as Professor Press, Professor Smith and Professors Noel. Each is an instance of a teacher. The concept of an instance is very important to known relationships.

- Person: student, teacher
- Place: classroom, building
- Thing: computer, lab equipment
- Concept: course, student’s attendance

When you attempt to discover possible entities in your data model and it is frequently helpful to look (or  listen)  for  nouns  /  noun  phrases  during  the  requirements  analysis.  Since nouns describe people, places, things or concepts, this trick commonly leads you to an entity. The challenge is to distinguish among the relevant and the irrelevant concepts.   Let consider the following excerpt from the case study:

“We have ten departments on campus which provide about 400 different courses – everything from Commerce 101 to Zoology 410. Of course, we don’t have sufficient professors to teach every course every semester but we do manage to offer about 150 different courses each semester.”