# Mermaid ER Diagram Cheat Sheet

Entities, their columns, and the cardinalities between them.

## Entities and attributes

Curly braces hold the columns. The type comes first, then the name.

```mermaid
erDiagram
  CUSTOMER {
    string name
    string email
  }
  ORDER {
    int orderNumber
    date placedAt
  }
```

![Two entity boxes, Customer and Order, each listing two attributes in a table.](/diagrams/er/entities.svg)

## Keys

PK, FK and UK mark the primary, foreign and unique keys.

```mermaid
erDiagram
  CUSTOMER {
    string custNumber PK
    string email UK
  }
  ORDER {
    int orderNumber PK
    string custNumber FK
  }
```

![Two entity boxes whose attribute rows carry small key markers: PK, UK and FK.](/diagrams/er/keys.svg)

## Cardinality

The marks at each end say how many. A bar is one, a crow's foot is many.

```mermaid
erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ LINE_ITEM : contains
  PRODUCT }o--o{ TAG : tagged
```

![Three entity boxes joined by lines whose ends carry a bar on one side and a circle and a crow's foot on the other.](/diagrams/er/cardinality.svg)

## Labelling the relationship

The text after the colon is the verb between the two entities. More than one word needs quotes.

```mermaid
erDiagram
  CUSTOMER ||--o{ ORDER : "places an order"
  ORDER ||--|{ LINE_ITEM : contains
```

![Two entity boxes joined by lines labelled places an order and contains.](/diagrams/er/labels.svg)

## Draw it in Mernyx.

Open a .mmd file, move a node on the canvas, and the source comes with it. Native on macOS, no browser involved.