ER diagrams: cardinality and identifying relations

简介 / Introduction

ER diagrams (Entity-Relationship diagrams) describe entities, attributes, and their connections in a data model. Cardinality expresses the numerical constraint between two entities, while an Identifying Relationship distinguishes strong from weak entities. This post shows both PlantUML and Mermaid syntax with a side-by-side comparison.

PlantUML 版

PlantUML uses symbol pairs like ||--o{ to express cardinality and identifying relations:

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@startuml
entity "Customer" as C {
  +customer_id : int <<PK>>
  --
  name : varchar
}
entity "Order" as O {
  +order_id : int <<PK>>
  --
  customer_id : int <<FK>>
  amount : decimal
}
C ||--o{ O : places

entity "OrderItem" as OI {
  +order_id : int <<PK, FK>>
  +product_id : int <<PK>>
  quantity : int
}
O ||--|| OI : contains
@enduml

Mermaid 版

Mermaid uses single-character combinations for cardinality, with double bars || marking identifying relations:

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erDiagram
CUSTOMER ||--o{ ORDER : places
ORDER ||--|| ORDERITEM : contains
CUSTOMER {
    int customer_id PK
    varchar name
}
ORDER {
    int order_id PK
    int customer_id FK
    decimal amount
}
ORDERITEM {
    int order_id PK, FK
    int product_id PK
    int quantity
}

对比 / Side-by-side

PlantUML’s symbol system is more visual — line style directly conveys cardinality and entity strength. Mermaid’s compact letter combos are quicker to learn and easier to embed in Markdown.

小结 / Wrap-up

ER diagrams are a core tool in database design. Mastering cardinality and identifying relation syntax in both PlantUML and Mermaid lets you switch tools fluidly across documentation contexts. Pick PlantUML for richer visual expressiveness, or Mermaid for fast inline diagrams in Markdown.