Entity Relationship (ER) diagram is a visual representation of entities and their relationships to each other in a database. It is used to design and represent a database schema, which is a blueprint of a database. ER diagrams are used by database designers to model and design databases in a structured manner. In this tutorial, we will go through the basics of creating an ER diagram.
Components of an ER Diagram
There are three main components of an ER diagram:
- Entities: An entity is a person, place, thing, or event that we want to store information about in the database. For example, a customer, an employee, a product, etc.
In the context of Entity-Relationship (ER) diagrams in DBMS, an entity is a real-world object or concept that has a unique identity and can be distinguished from other objects. An entity is represented as a rectangle in an ER diagram and contains attributes that describe its properties. For example, in a university database, student, course, and professor can be entities.
A weak entity, on the other hand, is an entity that cannot be uniquely identified by its attributes alone. It is always associated with another entity called the owner entity. The weak entity’s existence depends on the existence of the owner entity. A weak entity is represented as a rectangle with double lines in an ER diagram, and a diamond symbol represents the relationship between the owner entity and the weak entity.
For example, consider a bank database where a customer can have multiple accounts. In this case, the account entity is dependent on the customer entity, and it cannot exist without a corresponding customer. Thus, the account entity is a weak entity, and the customer entity is the owner entity. The account entity would have attributes such as account number, account type, and balance, while the customer entity would have attributes such as customer ID, name, and address. The relationship between the two entities would be represented by a diamond symbol, and the attributes of the account entity would be underlined to signify its weakness.
2. Attributes: In an entity-relationship (ER) diagram, attributes represent the characteristics or properties of an entity. There are different types of attributes in an ER diagram, including key attributes, composite attributes, multivalued attributes, and derived attributes.
Attribute: An attribute is a characteristic or property of an entity. For example, in an ER diagram for a student database, the “name” and “age” of the student are attributes of the “student” entity.
Key Attribute: A key attribute is an attribute that uniquely identifies an entity. In other words, it is a unique identifier of the entity. For example, in the student database, the “student ID” can be a key attribute that uniquely identifies each student.
Composite Attribute: A composite attribute is an attribute that can be further divided into smaller sub-attributes. For example, in the student database, the “address” attribute can be further divided into “street,” “city,” and “zip code” sub-attributes.
Multivalued Attribute: A multivalued attribute is an attribute that can have multiple values for a single entity. For example, in the student database, the “hobby” attribute can have multiple values such as “reading,” “sports,” and “music” for a single student.
Derived Attribute: A derived attribute is an attribute that is derived or calculated from other attributes. For example, in the student database, the “age” attribute can be a derived attribute that is calculated based on the “birth date” attribute.
It is important to properly identify and define the attributes in an ER diagram to accurately represent the relationships and constraints between entities.
3. Relationships: Relationships are the links between entities. They show how entities are related to each other. For example, a customer may place an order for a product, which creates a relationship between the customer, order, and product entities.
Creating an ER Diagram
To create an ER diagram, follow the steps below:
Step 1: Identify the entities Identify the entities in your database and list them down. For example, if you are designing a database for a hospital, the entities may include patients, doctors, nurses, etc.
Step 2: Identify the attributes For each entity, identify the attributes. For example, the patient entity may have attributes like name, date of birth, gender, etc.
Step 3: Identify the relationships Identify the relationships between the entities. For example, a patient may be treated by one or more doctors, creating a relationship between the patient and doctor entities.
Step 4: Create the ER diagram Using a tool such as Microsoft Visio, create the ER diagram by dragging and dropping the entities, attributes, and relationships onto the canvas. Connect the entities with relationships and add cardinality and participation constraints.
Cardinality and Participation Constraints
Cardinality constraints define the number of instances of one entity that can be related to another entity. There are three types of cardinality constraints:
One-to-One (1:1): Each instance of one entity is related to only one instance of another entity.
One-to-Many (1:N): Each instance of one entity is related to one or many instances of another entity.
When more than one instance of the entity on the left, and only one instance of an entity on the right associates with the relationship then it is known as a many-to-one relationship.
Many-to-Many (N:M): Each instance of one entity is related to many instances of another entity, and vice versa.
Participation constraints define whether an instance of an entity must participate in a relationship. There are two types of participation constraints:
- Mandatory: Each instance of an entity must participate in a relationship.
- Optional: Each instance of an entity may or may not participate in a relationship.
Here are some additional points on ER diagrams:
- ER diagrams can also be used to represent various types of relationships between entities, such as one-to-one, one-to-many, and many-to-many relationships. These relationships can be indicated using various symbols and connectors, such as crow’s foot notation.
- In addition to entities and relationships, ER diagrams can also include attributes, which are specific pieces of information that describe an entity. Attributes can be simple (i.e. single-valued) or composite (i.e. made up of multiple sub-attributes), and can also be categorized as either mandatory (i.e. must have a value) or optional (i.e. may or may not have a value).
- Another important concept in ER diagrams is cardinality, which refers to the minimum and maximum number of times that an entity can be associated with another entity in a given relationship. For example, a one-to-many relationship between a customer entity and an order entity would have a cardinality of (1, N), indicating that each customer can have multiple orders, but each order can only belong to one customer.
- ER diagrams can also be used to help identify and resolve design issues in a database. For example, a poorly designed ER diagram may result in data duplication, inconsistencies, or other problems. By carefully designing an ER diagram and ensuring that it accurately represents the relationships between entities, developers can help ensure that the database is properly normalized and optimized for performance.
An ER diagram is an important tool for designing a database schema. It helps in visualizing the relationships between entities and their attributes, and in understanding the cardinality and participation constraints. By following the steps outlined in this tutorial, you can create an effective ER diagram for your database.
Notation of ER diagram
Database can be represented using the notations. In ER diagram, many notations are used to express the cardinality. These notations are as follows: