Entity-Relationship Model

Databases come in various sizes, comprising different numbers of tables and relationships. This is why we use diagrams when designing databases – they offer a visual representation of the database's structure.

Let's get acquainted with the most commonly used diagram in database design: the entity-relationship model (often abbreviated as ER model). For instance:

In this example, we have two entities: School and Student. The School entity boasts attributes like id, name, and location, whereas the Student entity features attributes such as id, name, and surname. There exists a relationship between the Student and School entities, which we label as "study".

In tables of relational databases, this can be depicted using a foreign key:

The "study" relationship is captured using a foreign key (the school_id column in this case).

Such models are instrumental in crafting a coherent database structure.

Concepts

The ER model is underpinned by three essential concepts:

• Entities: These can be tangible or abstract objects, like schools and students. Represented by a Rectangle;
• Attributes: These are features or properties tied to entities. For instance, a school might have attributes like location or founding year, while a student might have age or group. Represented by an Ellipse;
• Relationships: These signify the interactions or connections between entities or attributes. Represented by a Rhombus.

You can see the graphical representation of these elements in the image provided:

Modern Entity-Relationship Diagram Presentation

Given that attribute shapes can take up a significant amount of space on a diagram, there's an alternative representation:

In this format, you'll notice a single-column table. The header denotes the entity, while each row below corresponds to the attributes linked to that entity.