1 SELECT t1.* FROM t1 LEFT JOIN t2 ON t1.key = t2.key WHERE t2.key IS NULL

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2 SELECT t2.* FROM t1 LEFT JOIN t2 ON t2.key = t1.key WHERE t1.key IS NULL

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3SELECT t1.* FROM t1, t2 WHERE t1.key = t2.key AND NOT t1.content = t2.content

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4 SELECT t1.* FROM t1, t2 WHERE t1.key = t2.key AND t1.field1 = t2.field1 AND...