SELECT *,DATEDIFF(DAY,订单时间,支付时间) AS 时间差 FROM A WHERE DATEDIFF(DAY,订单时间,支付时间)>3
UNION
SELECT *,DATEDIFF(DAY,订单时间,支付时间) AS 时间差 FROM B WHERE DATEDIFF(DAY,订单时间,支付时间)>3
去除了重复的结果
SELECT *,DATEDIFF(DAY,订单时间,支付时间) AS 时间差 FROM A WHERE DATEDIFF(DAY,订单时间,支付时间)>3
UNION ALL
SELECT *,DATEDIFF(DAY,订单时间,支付时间) AS 时间差 FROM B WHERE DATEDIFF(DAY,订单时间,支付时间)>3