Class vs ER diagram: domain modeling

简介 / Introduction

Class diagrams and ER diagrams both describe structured data, but with different focuses: class diagrams target object-oriented design with classes, attributes, and methods; ER diagrams target database design with entities, attributes, and relationships. This post uses the same business scenario to demonstrate both PlantUML and Mermaid syntax, then compares them side by side.

PlantUML 版

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@startuml
class User {
  -id: Long
  -name: String
  -email: String
  +getName(): String
}
class Order {
  -id: Long
  -total: BigDecimal
  +calculateTotal(): BigDecimal
}
class Product {
  -id: Long
  -title: String
  -price: BigDecimal
}
User "1" --> "*" Order : places
Order "*" --> "*" Product : contains
@enduml

PlantUML uses the class keyword to define class blocks, - for private members, + for public, and (): ReturnType for methods. Relationships use A --> B with multiplicity labels.

Mermaid 版

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classDiagram
    class User {
        -id: Long
        -name: String
        -email: String
        +getName(): String
    }
    class Order {
        -id: Long
        -total: BigDecimal
        +calculateTotal(): BigDecimal
    }
    class Product {
        -id: Long
        -title: String
        -price: BigDecimal
    }
    User "1" --> "*" Order : places
    Order "*" --> "*" Product : contains

Mermaid’s classDiagram uses class with braces for member definitions. Access modifiers use - and + the same way. Relationships use --> with multiplicity; in HTML rendering --> must be escaped as -->.

对比 / Side-by-side

小结 / Wrap-up

Class diagrams suit object-oriented analysis and design, while ER diagrams are better for database schema design. PlantUML has a richer ecosystem for complex projects (skinparam, themes), and Mermaid embeds more easily in Markdown docs and web pages. Knowing both gives you flexibility when communicating designs.