SELECT TBL1.ID, TBL1.FIELD1
FROM TBL1 LEFT JOIN TBL2 ON TBL1.ID=TBL2.ID
WHERE TBL2.ID=NULL
UNION
SELECT TBL2.ID, TBL2.FIELD1
FROM TBL2 LEFT JOIN TBL1 ON TBL2.ID=TBL1.ID
WHERE TBL1.ID=NULL