Entity Relationship Diagram
An Entity-Relationship Diagram (ERD) is a visual representation of the relationships between entities in a database. It is a powerful tool used in software development and database design to model and understand the structure of a relational database.
There are many different notations and there is no standard. We are using the crows-feet notation. You can build your own ERD in Lucidchart.
Let's walk through an example database design problem and visualise it with ERD!
Design a system that stores the users and their notebooks. Each notebook has a name and description. Each notebook contains several notes (but can be empty) and each note has a title and contents. A user can own multiple notebooks but each notebook belongs to exactly one user, and each note belongs to exactly one notebook. Each user has a username, email, and password that they store.
Entities
In an Entity-Relationship Diagram (ERD), an entity represents a real-world object or concept. Entities can be classified as either strong or weak.
Properties
Entities have properties, also known as attributes, which describe their characteristics.
Primary Key(PK)
Foreign Key(FK)
Unique
...
Relationships
Relationships represent associations between entities. There are three types of relationships:
one-to-one (1-1)
A one-to-one relationship means that each instance of one entity is associated with exactly one instance of another entity.
many-to-one (M-1)
A many-to-one relationship means that multiple instances of one entity can be associated with a single instance of another entity.
many-to-many (M-M)
A many-to-many relationship means that multiple instances of one entity can be associated with multiple instances of another entity.
Crow's feet notation
Full ERD
An enhanced ERD (EERD) extends the capabilities of a traditional ERD by introducing additional concepts such as specialization/generalization, aggregation, and inheritance. These concepts allow for more complex modeling of relationships and behaviors in a database system.
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