Database schema diagrams

Use a database schema diagram to show the shape of your data: a relational schema drawn as an entity-relationship diagram. It uses Mermaid’s erDiagram. In the example, a customer places orders, each order contains order_items, and every line item points at a product. ORDER_ITEM is the join table that resolves the many-to-many between orders and products.

erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ ORDER_ITEM : contains
  PRODUCT ||--o{ ORDER_ITEM : ordered_as

  CUSTOMER {
    bigint id PK
    text email UK "lowercased on write"
    text name
    timestamptz created_at
  }
  ORDER {
    bigint id PK
    bigint customer_id FK
    text status "pending | paid | shipped | cancelled"
    numeric total
    timestamptz created_at
  }
  PRODUCT {
    bigint id PK
    text sku UK
    text name
    numeric price
    int stock
  }
  ORDER_ITEM {
    bigint id PK
    bigint order_id FK
    bigint product_id FK
    int quantity
    numeric unit_price
  }

The first keyword is erDiagram.

• Relationships read A <card><line><card> B : label. The crow’s-foot cardinality markers sit at each end: |o zero-or-one, || exactly-one, }o zero-or-many, }| one-or-many, mirrored on the near side as o|, ||, o{, |{. <line> is -- for an identifying relationship or .. for non-identifying. The label names the relationship. For example, CUSTOMER |o..o{ NOTE : "may leave" is an optional, non-identifying link.
• Each ENTITY { … } block lists columns as type name [key]. Column tags render as badges (PK, FK, UK), and a quoted string after a column becomes an info note.

The same web shop is modelled as an object graph and a lifecycle on the Class & State page. To colour a table, use a vizzy-fence style override. Mermaid’s own style/classDef only apply inside flowcharts.