# 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](https://www.lucidchart.com).

**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](https://prepinsta.com/dbms/weak-entity-and-strong-entity/).

![ERD 1.0](https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-e1b1f3b723ab3b88f2a7a99f9c38589b27cca34e%2Fimage.png?alt=media)

## Properties

Entities have properties, also known as attributes, which describe their characteristics.

* Primary Key(PK)
* Foreign Key(FK)
* Unique
* ...

![ERD 2.0](https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-83750c07be9db5cb8ba5bfc8d0f72e0a608aba28%2Fimage-2.png?alt=media)

## 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

![Crow's Feet Notation](https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-5ceadb9ded732b63d8aed1e2935d233072580f8c%2Fimage-3.png?alt=media)

## Full ERD

![Full ERD](https://2807223923-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FTUqAJOgHs57S8lmqdxRV%2Fuploads%2Fgit-blob-6867874b2acf32f3d7ceca2db7acfbc61bff7727%2Fimage-4.png?alt=media)

## [Enhanced ERD](https://www.lucidchart.com/pages/enhanced-entity-relationship-diagram)

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.