Skip to main content
An entity-relationship (ER) model describes interrelated things of interest in a specific domain of knowledge. An ER model is composed of entity types and specifies relationships that can exist between entities.

​
Basic example

Entity names are often capitalized, although this is not required in Mermaid.

​
Entities with attributes

Define attributes with their types:

​
Syntax

​
Entities and relationships

Each statement consists of: 
<first-entity> [<relationship> <second-entity> : <relationship-label>]
Example: This reads as: “A property contains one or more rooms, and a room is part of one and only one property.”

​
Relationship syntax

The relationship consists of three parts:
  • Cardinality of the first entity
  • Whether the relationship confers identity
  • Cardinality of the second entity

​
Cardinality

Value (left)Value (right)Meaning
|oo|Zero or one
||||Exactly one
}oo{Zero or more (no upper limit)
}||{One or more (no upper limit)
Aliases like one or more, zero or many, 1+, 0+ are also supported.

​
Identification

Relationships can be identifying (solid line) or non-identifying (dashed line): Using aliases:

​
Attributes

Define attributes with type and name:

​
Attribute keys and comments

Specify primary keys, foreign keys, and comments:
Available keys: PK (Primary Key), FK (Foreign Key), UK (Unique Key). Multiple keys can be specified with commas.

​
Entity name aliases

Use aliases for display names:

​
Unicode and Markdown

​
Unicode text

​
Markdown formatting

​
Direction

Set the diagram orientation:

​
Top to bottom

​
Left to right

  • TB - Top to bottom
  • BT - Bottom to top
  • RL - Right to left
  • LR - Left to right

​
Styling

​
Styling a node

​
Using classes

With relationships:

​
Configuration

​
Layout

For larger diagrams, use ELK layout:
Requires Mermaid version 9.4+ with lazy-loading enabled.

Build docs developers (and LLMs) love

Get started for freeTalk to us