最小差额：
select top 1 abs(t1.i-t2.i) d
from t t1, a t2
where t1.i<>t2.i
order by d

最大差额：
select top 1 abs(t1.i-t2.i) d
from t t1, a t2
where t1.i<>t2.i
order by d desc