定义: 在Y函数依赖于X函数的基础上,即X→YX\to YX→Y时,如果Y⊂XY \subset XY⊂X,那么称X→YX\to YX→Y是平凡的函数依赖,
如果有Y⊂XY \subset XY⊂X,那么X→YX\to YX→Y一定成立
原因:因为Y⊂XY \subset XY⊂X,那么Y必然是X中的一部分,因为X确定了,那么自然其子集也确定了,整体可以决定部分
整体可以决定部分
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