Our schema 

CREATE TABLE comment (
   comment_id          INTEGER   PRIMARY KEY,
   thread_id           INTEGER   NOT NULL,
   parent_comment_id   INTEGER,
   score               INTEGER   NOT NULL DEFAULT 0,
   text                TEXT      NOT NULL,
   created             TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,

   CONSTRAINT FK_parent_comment
      FOREIGN KEY (parent_comment_id) REFERENCES comment (comment_id)
);