Entity Relationship Diagram

Based on the Information Engineering notation.

This is an extension to the existing Class Diagram. This extension adds:

Additional relations for the Information Engineering notation.
An entity alias that maps to the class diagram class.
An additional visibility modifier * to identify mandatory attributes.

Otherwise, the syntax for drawing diagrams is the same as for class diagrams. All other features of class diagrams are also supported.

Information Engineering Relations

Type Symbol
Zero or One |o--
Exactly One ||--
Zero or Many }o--
One or Many }|--

Examples:

@startuml

Entity01 }|..|| Entity02
Entity03 }o..o| Entity04
Entity05 ||--o{ Entity06
Entity07 |o--|| Entity08

@enduml

Entities

@startuml
entity Entity01 {
  * identifying_attribute
  --
  * mandatory_attribute
  optional_attribute
}
@enduml

Again, this is the normal class diagram syntax (aside from use of entity instead of class). Anything that you can do in a class diagram can be done here.

The * visibility modifier can be used to identify mandatory attributes. A space can be used after the modifier character to avoid conflicts with the creole bold:

@startuml
entity Entity01 {
   optional attribute
   **optional bold attribute**
   * **mandatory bold attribute**
}
@enduml

Complete Example

@startuml

' hide the spot
hide circle

' avoid problems with angled crows feet
skinparam linetype ortho

entity "Entity01" as e01 {
  *e1_id : number <<generated>>
  --
  *name : text
  description : text
}

entity "Entity02" as e02 {
  *e2_id : number <<generated>>
  --
  *e1_id : number <<FK>>
  other_details : text
}

entity "Entity03" as e03 {
  *e3_id : number <<generated>>
  --
  e1_id : number <<FK>>
  other_details : text
}

e01 ||..o{ e02
e01 |o..o{ e03

@enduml

Currently the crows feet do not look very good when the relationship is drawn at an angle to the entity. This can be avoided by using the linetype ortho skinparam.

Type	Symbol
Zero or One	`\|o--`
Exactly One	`\|\|--`
Zero or Many	`}o--`
One or Many	`}\|--`