#! pesudocode

Entity {
  id integer PRIMARY KEY,
  name varchar,
  ...
}

Play {
  id integer PRIMARY KEY REFERENCES Entity(id),
  director varchar,
  ...
}

Concert {
  id integer PRIMARY KEY REFERENCES Entity(id),
  start datetime,
  venue varchar,
  band varchar,
  ...
}

Comment {
  id integer PRIMARY KEY,
  entity integer REFERENCES Entity(id),
  comment_text varchar,
  ...
}