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SELECT 'different', a.field1, b.field1, a.field2, b.field2
FROM a, b
WHERE a.key1 = b.key1
  AND a.key2 = b.key2;

SELECT 'missing in b', a.*
FROM a LEFT OUTER JOIN b
  ON a.key1 = b.key1, a.key2 = b.key2
WHERE b.key1 IS NULL AND b.key2 IS NULL