ER Diagram Maker.

What is an ER diagram?

An entity relationship diagram (ERD) describes the data a system stores: the entities, such as customers or orders, their attributes, and how entities relate to each other. Developers use ERDs to design a database schema before writing migrations, and to explain an existing schema to new team members.

This ER diagram tool draws the diagram from a compact text definition, using crow’s foot notation for cardinality.

Crow’s foot notation cheat sheet

A relationship is written LEFT <left marker>--<right marker> RIGHT : label. Each marker describes how many records can be on that side:

• CUSTOMER ||--o{ ORDER : places — one customer places zero or more orders.
• ORDER ||--|{ ORDER_ITEM : contains — every order contains at least one line.
• USER ||--|| PROFILE : owns — one-to-one.
• Two dashes -- draw an identifying relationship (solid line); two dots .. draw a non-identifying one (dashed line).

Attributes and keys

erDiagram
    PRODUCT {
        int id PK
        string sku UK "Stock keeping unit"
        decimal price
        int category_id FK
    }

Each attribute line is a type, a name, optional keys — PK primary key, FK foreign key, UK unique — and an optional comment in quotes. Types are free text, so you can use the names of your database, such as varchar, uuid or timestamptz.

Design a database schema step by step

1. List the nouns of your domain: customers, products, orders, invoices. Each becomes an entity.
2. Give every entity a primary key and the attributes you really need.
3. Draw the relationships and decide each cardinality by asking “can there be none? can there be many?” on both sides.
4. Replace every many-to-many relationship with a junction entity, such as ORDER_ITEM between ORDER and PRODUCT, and add its foreign keys.
5. Review names: singular entity names, consistent snake_case attributes and clear relationship labels.
6. Export the diagram for the design document and keep the source next to your migrations.

The diagram does not connect to a database and does not generate or import SQL. To create SQL inserts from sample data, see JSON to SQL and CSV to SQL.

Download SVG for documents, websites and slides that need sharp lines at any size, or PNG at 1× to 4× resolution for chat apps, tickets and tools that do not accept SVG. The background can match the theme, be white or be transparent.

Rendering happens in your browser with the open-source Mermaid library. Your code is not uploaded, drafts are kept in this browser’s local storage, and a share link stores the compressed diagram after the # sign, which browsers do not send to the web server.