if x->a and y-> b
then
from x->a == xy-->ay [ir2] xy-->a [ir4]
from y->b == xy-->xb [ir2] xy-->b [ir4]
therefore xy-->ab [ir5]
但是 elmasri navathe 说,x-->a 和 y-->b 并不意味着 xy-->ab 我只是从功能依赖开始,所以有人能指出我缺少什么吗?