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SELECT (t1.nid + 1) as gap_starts_at,
       (SELECT MIN(t3.nid) -1 FROM node t3 WHERE t3.nid > t1.nid) as gap_ends_at 
FROM node t1
  WHERE NOT EXISTS (SELECT t2.nid FROM node t2 WHERE t2.nid = t1.nid + 1) 
  HAVING gap_ends_at IS NOT NULL