@startuml
!define Table(name,desc) class name as "desc" << (T,#FFAAAA) >>
!define primary_key(x) <b>x</b>
!define unique(x) <color:green>x</color>
!define not_null(x) <u>x</u>
hide methods
hide stereotypes

' entities
Table(group, "group") {
  primary_key(id) INTEGER
  not_null(name) VARCHAR[255]
}

' relationships
group --> group : "A group may be a sub group of another group"
@enduml