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#! pesudocode

...
  comment_text varchar,
  ...
}

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

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

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

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