An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity.
An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values. For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint.
Entities are represented by means of their properties, called attributes. All attributes have values. For example, a student entity may have name, class, and age as attributes.
There exists a domain or range of values that can be assigned to attributes. For example, a student’s name cannot be a numeric value. It has to be alphabetic. A student’s age cannot be negative, etc.
Types of Attributes
These attribute types can come together in a way like −
- simple single-valued attributes
- simple multi-valued attributes
- composite single-valued attributes
- composite multi-valued attributes
Entity-Set and Keys
Key is an attribute or collection of attributes that uniquely identifies an entity among entity set.
For example, the roll_number of a student makes him/her identifiable among students.
- Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.
- Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key.
- Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.
The association among entities is called a relationship. For example, an employee works_at a department, a student enrolls in a course. Here, Works_at and Enrolls are called relationships.
Degree of Relationship
The number of participating entities in a relationship defines the degree of the relationship.
- Binary = degree 2
- Ternary = degree 3
- n-ary = degree
Cardinality defines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set.
- One-to-one − One entity from entity set A can be associated with at most one entity of entity set B and vice versa.
One-to-many − One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.
Many-to-one − More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.
Many-to-many − One entity from A can be associated with more than one entity from B and vice versa.