多分支处理

[best_boy]

a1,a2为一表中的数字型字段(内部关系a1<a2)
b1,b2为另一表中的数字型字段(b1<b2)
当b2<a1或b1>a2时,不执行操作
当b1<=a1且b2<=a2时,t=b2-a1
当b1>a1且b2>a2时,t=a2-b1

怎么实现啊

[best_boy]

b1,b2(b1<b2)与a1,a2(a1<a2)相比应该有几种情况啊?
是不是5种啊

1.b2<a1
2.b1>a2
3.b1<=a1 and a1<b2<=a2
4.a2>b1>=a1 and b2>a2
5.a1<=b1 and b2<=a2

用if语句写是不是很繁锁啊
select case可以写吗?

[「已注销」]

if b1<=a1 and b2<=a2 then
t=b2-a1
elseif b1>a1 and b2>a2 then
t=a2-b1
end if