Mermaid ER diagram syntax: entities, attributes and cardinality with examples

An entity-relationship diagram is the schema picture: the things a database stores, the columns on each, and how rows point at each other. Mermaid's erDiagram draws it with crow's-foot cardinality markers, the convention data modellers already read.

Everything on this page is rendered by makesPDF's own Mermaid renderer, so a ```mermaid block produces exactly this in the PDF.

Entities and relationships

An entity is a bare name. A relationship is ENTITY_A <cardinality>--<cardinality> ENTITY_B : label, and it declares both entities if they do not exist yet:

erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ LINE_ITEM : contains

Read a marker from the entity it sits against: || is exactly one, o{ is zero or more, |{ is one or more, o| is zero or one. The line is solid for an identifying relationship and dotted — .. — for a non-identifying one.

Attributes

A { … } block after the entity lists its attributes, one per line as type name. Order is the type first: string name, int id:

erDiagram
  USER {
    int id PK
    string name
    string email
    date created_at
  }
  ORDER {
    int id PK
    int user_id FK
    string status
    decimal total
  }
  USER ||--o{ ORDER : places

The trailing keyword marks the attribute's key role — PK primary key, FK foreign key, UK unique — and is drawn beside the type.

Cardinality in practice

The four shapes cover the relationships that matter, and they combine in both directions:

erDiagram
  AUTHOR ||--o{ BOOK : writes
  BOOK ||--o{ REVIEW : receives
  USER ||--o| AVATAR : has
  TEAM }o--o{ USER : "member of"

}o--o{ is the many-to-many form — the marker on each end is read the same way, just mirrored. A quoted label is required when it contains spaces: : "member of".

A fuller schema

Attribute blocks and relationships can be interleaved in any order, and an entity can be described in one block while its relationships are declared elsewhere:

erDiagram
  INVOICE {
    string id PK
    string customer_id FK
    date issued
    decimal amount
  }
  PAYMENT {
    string id PK
    string invoice_id FK
    string method
  }
  INVOICE ||--o{ PAYMENT : "settled by"
  INVOICE }o--|| CUSTOMER : "billed to"

Not supported in makesPDF yet: attribute comments — the quoted free text some Mermaid examples put after a key — are parsed but not drawn. Entity-level styling is not applied, so every entity uses the theme's fill.