What cardinality symbols does Mermaid use for ER diagrams?

Mermaid uses crow's foot notation: `||--||` for exactly-one to exactly-one, `||--o{` for exactly-one to zero-or-many, `}o--o{` for zero-or-many to zero-or-many, and `||--|{` for exactly-one to one-or-many. These render as standard crow's foot symbols on the relationship lines, making cardinality immediately readable.

How do I mark primary keys and foreign keys in Mermaid ER diagrams?

Add `PK` or `FK` after the column type in the entity definition: `int id PK` for a primary key, `int userId FK` for a foreign key. You can also add `UK` for unique keys. These annotations appear as labels inside the entity box and are essential for communicating the relational structure of the schema.

Can I document a many-to-many relationship in Mermaid ER diagrams?

Yes. Represent many-to-many relationships directly with `}o--o{` or via a junction entity. Using a junction entity (like ORDER_ITEM between ORDER and PRODUCT) is usually better practice as it models real schema design more accurately and allows you to add attributes to the relationship itself, such as quantity or price.

What's the best way to use Mermaid ER diagrams in a project?

Add the ER diagram as a Mermaid code block in your project README or a dedicated `SCHEMA.md` file. GitHub, GitLab, and Notion all render Mermaid natively, so the diagram stays up to date with the code. Update the diagram whenever you run a migration — treating it as living documentation rather than a one-time artifact.

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Entity Relationship (ER) diagrams are the standard notation for visualizing relational database schemas. They show entities (tables), their attributes (columns), and the relationships between them — one-to-one, one-to-many, many-to-many. Mermaid's `erDiagram` syntax brings ER diagramming into the text-as-code world, making it easy to document database schemas in READMEs, ADRs (Architecture Decision Records), and pull request descriptions. When database engineers or backend developers ask AI assistants to design a schema — 'Create a database for an e-commerce platform with users, products, and orders' — the response often includes a Mermaid ER diagram. Pasting that code into Zolkit lets you immediately validate that the relationships and cardinalities reflect the intended business rules. Zolkit's ER diagram viewer fully supports Mermaid's `erDiagram` syntax including entity definitions with typed attributes, primary and foreign key annotations, relationship lines with crow's foot notation, and inline relationship labels. It's used by backend developers, database architects, and technical writers to document schemas without leaving the browser.

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