When you want to model how data entities relate to each other in a system, Entity Relationship (ER) diagrams are the standard tool. With Mermaid.js, you can quickly create ER diagrams using a simple text-based syntax. In Mermaid, an entity represents a table or object, typically shown as a rectangle with the entity's name. Attributes are the pieces of data that belong to an entity, and relationships describe how entities are connected.

To define entities in Mermaid, use the erDiagram keyword. Each entity is declared by its name, followed by curly braces containing its attributes. Relationships between entities are specified on separate lines, using a line with a label to show the type of relationship, such as one-to-many or many-to-many connections.

In this example, you see two entities: BOOK and AUTHOR, each with their own attributes. The double bar and circle notation (||--o{) defines the cardinality of the relationship. Mermaid uses different line styles and symbols to indicate how many instances of one entity relate to another. For example, ||--o{ means one book can be written by many authors, and one author can write many books, representing a many-to-many relationship.

Entities are shown as rectangles with their attributes listed inside. Relationships are drawn as lines between entities, with a label describing the nature of the connection. By adjusting the symbols on the lines, you can model one-to-one, one-to-many, or many-to-many relationships. This makes ER diagrams in Mermaid both expressive and easy to read, helping you visualize how your data is structured and connected.