#Entity Relationship Diagram

An entity relationship diagram describes a data model: the entities (things a system stores) and the relationships between them, with cardinalities on each end. kymo's editor reads the Mermaid erDiagram syntax.

This page works like a quickstart: as you scroll, the pane on the right shows the source and the preview for the section you're reading. Copy grabs the source; ▶ Open in editor loads it into editor.kymo.studio (pick mermaid in the diagram-type dropdown when starting from scratch).

The example on the right is the classic order model: a statement is ENTITY <cardinality>--<cardinality> ENTITY : label, and the label names the relationship from the first entity's point of view.

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses

▶ Try it in the editor

#Cardinality

Each end of the relationship operator carries a two-character cardinality marker, read outward from the dashes:

Left	Right	Meaning
|o	o|	zero or one
||	||	exactly one
}o	o{	zero or more
}|	|{	one or more

The dashes in the middle encode identification: -- is an identifying relationship (the child cannot exist without the parent — drawn solid), .. is non-identifying (drawn dashed).

erDiagram
    CAR ||--o{ NAMED-DRIVER : allows
    PERSON }o..o| NAMED-DRIVER : is
    HOUSE |o--|| ADDRESS : has

▶ Try it in the editor

#Attributes

A { … } block after an entity lists its attributes as type name pairs. A trailing key keyword marks PK (primary key), FK (foreign key), or UK (unique key) — combine them with commas (PK, FK) — and a trailing quoted string adds a comment.

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string name
        string custNumber PK
        string sector
    }
    ORDER {
        int orderNumber PK
        string deliveryAddress FK "delivered to"
    }

▶ Try it in the editor

Status. ER diagram previews on this page and in the editor use the Mermaid renderer; importing ER diagrams into kymo's own pipeline (native SVG/PNG/PDF rendering) is on the roadmap.

#See also

• Class Diagram — for modelling behaviourful objects rather than stored data.
• Best Practices — layout and readability guidance.