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ER diagram

Mermaid keyword: erDiagram

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Sample

erDiagram
  USER ||--o{ POST : writes
  USER ||--o{ COMMENT : writes
  POST ||--o{ COMMENT : has
  POST }o--|| CATEGORY : belongs_to
  USER {
    int id PK
    string email
    string display_name
  }
  POST {
    int id PK
    string title
    string body
    int author_id FK
    int category_id FK
  }

When to reach for an ER diagram

ER diagrams are made for database schemas: tables, columns, primary keys, foreign keys, and the cardinality of relationships. If the audience needs to understand “one user has many posts” or “every comment must belong to a post”, an ER diagram says it on one screen.

For object-graph relationships in code (where types share inheritance), use a class diagram instead.

Cheat sheet

erDiagram
  A ||--o{ B : "has"   # A has many optional B
  A ||--|{ B : "has"   # A has one or more B (mandatory)
  A }|--|| B : "owned by"  # many A owned by exactly one B
  A {
    int id PK
    string name
    int b_id FK
  }

Common mistakes

1. Forgetting PK / FK markers. Without them the diagram is just a list of column names. Mark keys explicitly.
2. Listing every column. ER diagrams are most useful with the columns that matter to the relationship — usually the keys plus one or two human-readable fields.

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