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Entity relationship syntax

Learn relationships, cardinality markers, and attribute blocks used in the Simple data model template.

On this page9 sections
  1. Syntax cheatsheet
  2. 1.Start an ER diagram
  3. 2.Cardinality markers
  4. 3.Identifying vs non-identifying
  5. 4.Entity attributes
  6. 5.Keys and foreign references
  7. 6.Multiple relationships
  8. 7.Put it together: Simple data model
  9. Check your level

Syntax cheatsheet

Quick reference for this diagram type

ConceptSyntax
DeclareerDiagram
Exactly one||
Zero or one|o / o|
One or more}| / |{
Zero or more}o / o{
Identifying||--o{
Non-identifying}o..o{
RelationshipUSER ||--o{ FOLDER : owns
AttributesENTITY { type name PK }
  1. 1

    Start an ER diagram

    Begin with erDiagram. Entities are usually UPPER_SNAKE or Pascal names connected by relationship lines.

    Each relationship needs a left cardinality, a line, a right cardinality, and a label after the colon.

    erDiagram
  USER ||--o{ FOLDER : owns
    Expand
  2. 2

    Cardinality markers

    Cardinality describes how many instances participate. || is exactly one, o| zero or one, |{ one or more, o{ zero or more.

    Read USER ||--o{ FOLDER : owns as “one user owns zero or more folders.”

    • Tip: }o and |o appear on the opposite side of the line for the same meanings — pick a consistent left/right style.
    erDiagram
  USER ||--o{ DIAGRAM : owns
  FOLDER ||--o{ DIAGRAM : contains
  TEAM ||--|{ MEMBER : includes
    Expand
  3. 3

    Identifying vs non-identifying

    A solid relationship line (--) is identifying: the child depends on the parent’s identity.

    A dashed line (.. ) is non-identifying: entities are related but independently identified. Use solid for ownership/containment, dashed for looser references.

    erDiagram
  FOLDER ||--o{ DIAGRAM : contains
  USER }o..o{ DIAGRAM : favorites
    Expand
  4. 4

    Entity attributes

    List fields inside curly braces under an entity. Write type then name, one field per line.

    Keep types simple (string, int, date) unless you need domain-specific precision.

    • Tip: Mark keys with PK, FK, or UK after the attribute name. Multiple keys can be comma-separated (PK,FK).
    erDiagram
  USER {
    string id
    string email
  }
  FOLDER {
    string id
    string name
  }
    Expand
  5. 5

    Keys and foreign references

    Attributes support key constraints: PK (primary), FK (foreign), and UK (unique). Separate multiple keys with commas.

    Add an optional comment in double quotes after the keys to document the field.

    • Tip: Optional types end with ? on the type (string? nickname). Primary-key names may start with *.
    erDiagram
  FOLDER ||--o{ DIAGRAM : contains
  FOLDER {
    string id PK
  }
  DIAGRAM {
    string id PK
    string folderId FK
    string title
  }
    Expand
  6. 6

    Multiple relationships

    One entity can participate in many relationships. USER may own folders and diagrams while FOLDER contains diagrams.

    Draw each relationship on its own line; don’t try to compress them into one statement.

    erDiagram
  USER ||--o{ FOLDER : owns
  USER ||--o{ DIAGRAM : owns
  FOLDER ||--o{ DIAGRAM : contains
    Expand
  7. 7

    Put it together: Simple data model

    Combine ownership, containment, and attribute blocks into the examples template model.

    erDiagram
  USER ||--o{ FOLDER : owns
  USER ||--o{ DIAGRAM : owns
  FOLDER ||--o{ DIAGRAM : contains
  USER }o..o{ DIAGRAM : favorites
  USER {
    string id PK
    string email UK
  }
  FOLDER {
    string id PK
    string name
  }
  DIAGRAM {
    string id PK
    string folderId FK
    string title
    string visibility
  }
    Expand

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