TryMermaid

Mermaid ER Diagram Maker

Write ER diagrams in Mermaid code and watch them render as you type. Start from the example below, then export PNG or SVG, share a link or embed the diagram.

Updated · Code editor with live preview

ER diagram example

Rendering preview…

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : "appears in"
    CUSTOMER {
        string id PK
        string email
    }
    ORDER {
        string id PK
        string customer_id FK
        date created_at
    }
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What is an ER diagram?

An entity relationship (ER) diagram shows the entities in a data model, usually database tables, along with their attributes and the relationships between them, such as one-to-many. In Mermaid an ER diagram starts with the erDiagram keyword.

When to use an ER diagram

  • Designing a database schema
  • Documenting tables, primary keys and foreign keys
  • Reviewing data model changes in pull requests
  • Explaining how records relate to non-technical teammates

How to make an ER diagram in Mermaid

  1. Step 1

    Open the editor

    Open TryMermaid with the example on this page, or start a new diagram with erDiagram on the first line.

  2. Step 2

    Add relationships

    Write one relationship per line, such as CUSTOMER ||--o{ ORDER : places. The symbols on each side set the cardinality, and the text after the colon labels the relationship.

  3. Step 3

    Add attributes and keys

    Add a block after an entity name with one attribute per line, written as type name and optionally followed by PK, FK or UK.

  4. Step 4

    Export or share

    Check the live preview, then export the diagram as PNG or SVG or copy a share link.

ER diagram syntax cheat sheet

ElementSyntaxWhat it does
Exactly one||One and only one
Zero or one|o or o|Optional single
One or more}| or |{At least one
Zero or more}o or o{Any number
IdentifyingA ||--o{ B : labelSolid line
Non-identifyingA ||..o{ B : labelDashed line
Attributestring email UKType, name and optional key

ER diagram FAQ

How do I show a one-to-many relationship in Mermaid?

Put ||--o{ between the two entities, for example CUSTOMER ||--o{ ORDER : places. The || side means exactly one customer and the o{ side means zero or more orders. Use ||--|{ if every customer must have at least one order.

Can I mark primary and foreign keys?

Yes. Inside an entity block, add PK, FK or UK after the attribute name, for example string customer_id FK. You can combine keys with a comma, such as PK, FK, and add a quoted comment at the end of the line.

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