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name: entity-relationship-diagram description: "Turn a data model into an entity-relationship (ER) diagram. Use when asked to design a schema, model data, show how tables/entities relate, or diagram a database. Produces a ready-to-render Mermaid ER diagram (renders live, exportable as PNG/SVG) plus key attributes, cardinality, and design notes."

Entity-Relationship Diagram Skill

Before you write a migration, it pays to see the data model: the entities, their key fields, and how they relate (one-to-many, many-to-many). This skill turns a described domain into a clean Mermaid ER diagram with proper cardinality notation and the attributes that matter.

Required Inputs

Ask for these only if they aren't already provided:

• The entities — the core objects/tables (User, Order, Product…).
• Relationships — how they relate, and the cardinality (a user has many orders, an order has many line items).
• Key attributes — the important fields per entity (especially keys); full column lists aren't required.
• The domain — what the system does, so the model is realistic.

Output Format

[Domain] — data model

One line on the scope of the model.

erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : "appears in"
    USER {
        uuid id PK
        string email
        string name
    }
    ORDER {
        uuid id PK
        uuid user_id FK
        datetime created_at
        string status
    }
    LINE_ITEM {
        uuid id PK
        uuid order_id FK
        uuid product_id FK
        int qty
    }
    PRODUCT {
        uuid id PK
        string name
        decimal price
    }

Cardinality key — ||--o{ = one-to-many, }o--o{ = many-to-many, ||--|| = one-to-one.

Design notes — normalization choices, where a join table is needed, indexes worth adding, anything deferred.

Mermaid Rules (so it renders)

• Start with erDiagram. Relationship line: A ||--o{ B : label.
• Crow's-foot cardinality: || (exactly one), o{ (zero-or-many), |{ (one-or-many), o| (zero-or-one).
• Attribute blocks: ENTITY { type name PK } — mark keys with PK / FK.
• Entity names are usually UPPER_SNAKE; quote relationship labels that contain spaces.

Quality Checks

• Every relationship has explicit, correct cardinality (not just a plain line)
• Primary and foreign keys are marked (PK/FK)
• Many-to-many relationships are resolved with a join entity where appropriate
• Attribute types are sensible for the domain
• The Mermaid block renders without edits

Anti-Patterns

• Do not draw relationships without cardinality — "related" isn't a data model
• Do not leave many-to-many unresolved when a join table is the right call
• Do not dump every conceivable column — show the keys and the attributes that matter
• Do not omit foreign keys — they're how the relationships are actually enforced
• Do not break Mermaid with unquoted spaced labels

Based On

Data modeling (entity-relationship modeling, crow's-foot notation, normalization), expressed as renderable Mermaid.

Example Trigger Phrases

• "Design a schema."
• "Model data."
• "Show how tables/entities relate."
• "Diagram a database."