#! 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,
  ...
}

##</code><code>##

SELECT *
FROM Concert c
INNER JOIN Entity e ON c.id=e.id

##</code><code>##

SELECT *
FROM Play p
INNER JOIN Entity e ON p.id=e.id

##</code><code>##

SELECT *
FROM Comment k
INNER JOIN Entity e ON k.entity=e.id
INNER JOIN Play p ON k.entity=p.id

##</code><code>##

SELECT *
FROM Comment k
INNER JOIN Entity e ON k.entity=e.id
LEFT JOIN Play p ON k.entity=p.id
LEFT JOIN Concert c ON k.entity=c.id