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📖 Table of Contents

1. ER Model
   • Notation
   • Comments
2. Entity
   • Weak Entity
   • Inheritance
3. Attributes
   • Datatype
   • Classifier
4. Relationship
   • Degree
   • Cardinality
   • Participation
   • Roles

Last updated for: v0.3.0

ER Model

ER models are specified in .erd files with the erdiagram keyword and a model name. After the header follows an optional notation, before specification of the model elements (i.e. entities and relationships).

Syntax Graph (click to open)

// header (required)
erdiagram Name

// notation (optional)
notation=default

// model elements
entity E1 { id1 key }
entity E2 { id2 key }
relationship Rel { E1 -> E2 }  

Notation

The notation option changes the representation in the ER diagram to a specific notation.

Syntax Graph (click to open)

Notation Type	Description
default	Default Notation
chen	Chen Notation
bachman	Bachman Notation
crowsfoot	Crow's Foot Notation

See Notations for more information.

Comments

Single-line and multi-line comments are supported.

// single-line comment

/*
    multi-line comment
*/

Entity

An entity includes a name and attributes within curly braces.

Syntax Graph (click to open)

// Basic Entity
entity Basic {}

// Entity with Attributes
entity WithAttr { 
    attr1 key
    attr2
}

Weak Entity

The preceding weak keyboard defines a weak entity. While a key attribute is used in strong entities, weak entities use a partial-key for identification. The dependency is declared with an additional weak relationship.

entity StrongEntity {
    attr key
}

weak entity WeakEntity { 
    attr partial-key
}

weak relationship depends {
    WeakEntity -> StrongEntity
}

Inheritance

⚠️ (experimental)

entity Vehicle {
    manufacturer
    cost
}

entity Car extends Vehicle {
    serial_nr
}

Attributes

Syntax Graph (click to open)

// Basic
attr

// Datatype
attr: VARCHAR(255)
attr : integer

// Classifiers
attr key
attr partial-key 
attr derived

Datatype

Syntax Graph (click to open)

Classifier

Classifier	Description
none	Simple
key	Key attributes uniquely identify entities
partial-key	Partial key, used for weak entities
derived	Can be derived from another attribute (e.g., age from birthday)

Relationship

Syntax Graph (click to open)

Degree

relationship Binary {
    E1 -> E2
}

relationship Ternary {
    E1 -> E2 -> E3 
}

relationship Recursive {
    E1 -> E1
}

Cardinality

Cardinality corresponds to the maximum constraint in the number of allowed entity instances. The value for cardinality can be either 1 (one) or N (many).

// 1-1 (One-To-One)
relationship Rel1 {
    E1[1] -> E2[1]
}

// 1-N (One-To-Many)
relationship Rel2 {
    E1[1] -> E2[N]
}

// N-N (Many-To-Many)
relationship Rel3 {
    E1[N] -> E2[N]
}

Participation

Participation corresponds to the minimum constraint and can be either 0 or 1.

// zero-or-one (0..1) -> one-and-only-one (1..1)
relationship Rel1 {
    Entity1[0..1] -> Entity2[1..1]
}

// [1..1] is the same as [1] (one)
relationship Rel2 {
    Entity3[1..1] -> Entity4[1]
}

// zero-or-more (0..N) -> one-or-more (1..N)
relationship Rel3 {
    Entity5[0..N] -> Entity6[1..N]
}

// [1..N] is the same as [N] (many)
relationship Rel4 {
    Entity7[1..N] -> Entity8[N]
}

Roles

Roles can be specified after cardinality- and participation constraints.

relationship Rel {
    E1[ 1 | "role1" ] -> E2[ 1..N | "role2" ]
}