Entity Relationship

er Pirate Fleet

ships
  id int pk
  name varchar
  ship_type varchar
  cannons int
  1-aboard-* crew_members
  1-1 captains
  1-carries-* treasure

captains
  id int pk
  name varchar
  ship_id int fk
  bounty int
  ?-frequents-1 ports

crew_members
  id int pk
  name varchar
  ship_id int fk
  role varchar nullable

treasure
  id int pk
  name varchar
  value int
  ship_id int fk nullable

ports
  id int pk
  name varchar
  region varchar unique
  1-docks-* ships

Overview

ER diagrams render database-style entity-relationship boxes with columns, data types, and constraint icons. Tables connect via relationships with crow's foot cardinality markers. Layout is automatic via Dagre.

Syntax

er Diagram Title
notation crowsfoot

TableName (color)
  column_name type [constraints]

SourceTable 1--* TargetTable : label

Settings

Tables

Declare a table on an unindented line. Columns are indented below:

ships
  id int pk
  name varchar
  cannons int

Colors

Colors are auto-assigned from the palette. Override with a color name in parentheses:

ships (blue)
treasure (red)

Columns

Indented lines under a table are parsed as columns.

Format

  column_name type constraints

All parts except the name are optional:

  id int pk           // name + type + constraint
  name varchar        // name + type
  active nullable     // name + constraint
  notes               // name only

Constraints

Add constraints in square brackets. Multiple constraints are comma-separated:

  id int pk
  name varchar unique nullable
  ship_id int fk

Relationships

Relationships connect two tables with cardinality on each side.

Cardinality Values

Syntax

Use 1, *, or ? with dashes:

ships 1--* crew_members
ports ?--1 captains
ships 1--* treasure

Single dash also works:

ships 1-* crew_members

Labels

Add a label after a colon:

ships 1--* crew_members : sails on
ships 1--1 captains : commanded by

Notation Modes

Crow's Foot (default)

Cardinality is shown as graphical markers at each end of the relationship line:

• One (1): perpendicular bar
• Many (*): three-pronged fork
• Optional (?): circle with perpendicular bar

Labels Mode

Switch to text labels by adding notation labels:

er
notation labels

This replaces crow's foot markers with text like 1, *, and 0..1.

Comments

// This line is ignored by the parser

Complete Example

er Pirate Ports

ports (blue)
  id int pk
  name varchar
  region varchar unique

ships
  id int pk
  name varchar
  home_port int fk
  cannons int

captains (green)
  id int pk
  name varchar
  ship_id int fk
  bounty int

raids
  id int pk
  ship_id int fk
  port_id int fk
  date date
  plunder int nullable

ports 1--* ships : home of
ships 1--1 captains : commanded by
ships 1--* raids : launched
ports 1--* raids : targeted