ER diagrams: weak entities

简介 / Introduction

A weak entity is one that does not have a full primary key of its own and depends on a strong (owner) entity for its identification. Its primary key is composed of a partial key plus the foreign key referencing the owning entity. This article shows how to draw ER diagrams with weak entities in both PlantUML and Mermaid.

PlantUML 版

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@startuml
entity "Student" as Student {
  +studentId : int <<PK>>
  --
  name : varchar
  age : int
}

entity "Transcript" as Transcript <<WEAK>> {
  --
  courseId : int
  score : numeric
}

Student ||..o{ Transcript : "owns"
@enduml

Notes:

• <<WEAK>> marks the weak entity;
• + prefix denotes the primary key attribute;
• ||..o{ shows a “one owns many” relationship, where Student is the strong entity and Transcript depends on it.

Mermaid 版

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erDiagram
    Student {
        int studentId PK
        string name
        int age
    }
    Transcript {
        int courseId
        numeric score
    }
    Student ||--o{ Transcript : "owns"

Notes: Mermaid’s ER diagrams do not have a <<WEAK>> annotation; weak entity semantics are expressed implicitly through the relationship. ||--o{ denotes a one-to-many “owns” relationship.

对比 / Side-by-side

小结 / Wrap-up

PlantUML explicitly marks weak entities with <<WEAK>>, making the semantics clear. Mermaid expresses this implicitly via relationship arrows, offering simpler syntax but no dedicated weak-entity support. Choose based on your team’s tooling and documentation conventions.