说到功能依赖关系，A→B是否与B→A相同? [英] Is A → B the same as B → A when speaking of functional dependencies?

问题描述

这可能非常简单，但我只需要与你们确认一下即可.

This might very simple but I just had to check with you guys.

在数据库方面，文学中的箭头是否暗示着平等?

When it comes to databases, does the arrow in literature imply vise versa on equality?

意思是，A→B是否被视为与B→A相同，特别是在涉及数据库和功能依赖项时?

Meaning, is A → B considered the SAME as B → A, in particular when it comes to databases and functional dependencies?

推荐答案

请阅读有关FD(功能依赖项)的参考.

Please read the reference(s) you were given for FDs (functional dependencies).

FD是属性集A& A的形式"A→B"的表达式.B.因此，如果A和B不同，则A→B是与B→A不同的FD.

A FD is an expression of the form "A → B" for sets of attributes A & B. So if A and B are different, A → B is a different FD than B → A.

对于关系值或变量R，"A→B保留在R中"和"A→B保留在R中"表示如果两个R元组对A具有相同的子元组，那么它们对B就具有相同的子元组.

For a relation value or variable R, "A → B holds in R" and "A → B in R" say that if two R tuples have the same subtuple for A then they have the same subtuple for B.

R中的A→B是否等于R中的B→A?如果A和B是同一集合，则为是.但是，如果不是这样的话怎么办?

Is A → B in R equivalent to B → A in R? If A and B are the same set, then yes. But what if they aren't?

X Y
a 1
b 1

{X}→{Y}保持该关系值.{X}<> {Y}.{Y}→{X}也成立吗?

{X} → {Y} holds in that relation value. {X} <> {Y}. Does {Y} → {X} also hold?

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