The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered a good option for designing databases.
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
- Simple attribute− Simple attributes are atomic values, which cannot be divided further. For example, a student’s phone number is an atomic value of 10 digits.
- Composite attribute− Composite attributes are made of more than one simple attribute. For example, a student’s complete name may have first_name and last_name.
- Derived attribute− Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived. For another example, age can be derived from data_of_birth.
- Single-value attribute− Single-value attributes contain single value. For example − Social_Security_Number.
- Multi-value attribute− Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.
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.
A set of relationships of similar type is called a relationship set. Like entities, a relationship too can have attributes. These attributes are called descriptive attributes.
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
The E-R model can results problems due to limitations in the way the entities are related in the relational databases in a project. These problems are called connection traps. These problems often occur due to the incorrect name of the meaning of certain relationships.