Entity-relationship diagrams

Under the erd header, blocks declare entities and their fields, and statements between them declare relations. The connector is the cardinality, drawn in crow's-foot notation.

erd
title: Blog schema

users [blue] {
  id uuid pk
  email string
  team_id uuid fk
}
teams {
  id uuid pk
  name string
}
posts {
  id uuid pk
  author_id uuid fk
  title string
}

users N:1 teams: belongs to
users 1:N posts: writes

Entities and fields#

• Entity: Name [color] { fields }, or id = Name { … } for an explicit id. The opening { and closing } sit on their own lines, with one field per line between them.
• Field: name type key. The type and key are optional; a key is pk or fk. Field names with spaces use quotes.
• An empty entity (posts { }) is fine; its fields can be filled in later, or it can stand as a bare table.

erd
customers {
  id uuid pk
  "full name" string
  email string
}
orders {
  id uuid pk
  customer_id uuid fk
  total decimal
  placed_at timestamp
}
customers 1:N orders: places

Relations and cardinality#

The connector carries the cardinality. N:1 is sugar that swaps the ends, so it reads naturally either direction. A : label adds the verb.

Relations reference entities by name; entity.field references are accepted and resolve to the entity (the field part is documentation). Unlike graph nodes, a table must be declared: a relation to an undeclared entity is dropped with a diagnostic.

Precise crow's-foot marks#

The coarse words above can't tell "zero or many" from "one or many". For the full notation, write the crow's-foot glyphs directly: one mark per end, exactly as Mermaid does. Each end is one of:

Join two ends with -- (identifying, a solid line) or .. (non-identifying, a dashed line): customers ||--o{ orders is exactly-one to zero-or-many.

erd
orders {
  id uuid pk
}
line_items {
  id uuid pk
}
shipments {
  id uuid pk
}

orders ||--|{ line_items: contains
orders ||..o{ shipments: ships via

erd
users {
  id uuid pk
}
roles {
  id uuid pk
}
permissions {
  id uuid pk
}

users N:M roles: assigned
roles N:M permissions: grants

For a fuller picture, an e-commerce schema tying customers, orders, line items, and products together.

erd
title: Store schema

customers [blue] {
  id uuid pk
  email string
  created_at timestamp
}
orders [green] {
  id uuid pk
  customer_id uuid fk
  status string
  total decimal
}
line_items {
  id uuid pk
  order_id uuid fk
  product_id uuid fk
  quantity int
}
products [orange] {
  id uuid pk
  sku string
  name string
  price decimal
}

customers 1:N orders: places
orders 1:N line_items: contains
products 1:N line_items: sold as

Looking for classes and methods instead of tables? See UML class diagrams, which share this family but switch to UML boxes and relationship arrows.