部分依赖（数据库） [英] Partial Dependency(Databases)

问题描述

我需要关闭这个。
我制定了一个定义，部分依赖是当字段间接依赖于主键或部分依赖，但也依赖于依赖于主要的其他键，如果另一个字段依赖于id的字段删除该字段将仍然存在由于其对主键的依赖。我不知道如果它是正确的。

I need closure on this. I fabricated a definition that partial dependency is when fields are indirectly dependent on the primary key or partially dependent but are also dependent on other keys that depend on the primary such that if the field which another field depends on id deleted that field will still exist due to its dependence on the primary key. I am not sure if it is correct. I have researched and every definition sounds misleading.Is my definition correct and if not please explain?

推荐答案

一个函数依赖，它保存在

A functional dependency that holds in a relation is partial when removing one of the determining attributes gives a functional dependency that holds in the relation.

例如，如果{A，B}→{C}，而且还有{A} A}→{C}，则{C}部分地依赖于{A，B}。

Eg if {A,B} → {C} but also {A} → {C} then {C} is partially functionally dependent on {A,B}.

函数依赖X→Y一个完整的函数依赖如果
从X的任何属性A删除意味着依赖不会
持有更多;即，对于任何属性A∈X，（X - {A}）不
功能上确定Y.功能依赖性X→Y是部分
依赖性，如果某些属性A∈X可以被去除从X和
依赖关系仍然成立;对于某些AεX，（X - {A}）→Y。

A functional dependency X → Y is a full functional dependency if removal of any attribute A from X means that the dependency does not hold any more; that is, for any attribute A ε X, (X – {A}) does not functionally determine Y. A functional dependency X → Y is a partial dependency if some attribute A ε X can be removed from X and the dependency still holds; that is, for some A ε X, (X – {A}) → Y.

- 数据库系统的基础第六版Ramez Elmasri& Navathe

-- FUNDAMENTALS OF Database Systems SIXTH EDITION Ramez Elmasri & Navathe

（2NF的定义涉及密钥上非密钥属性的完全功能依赖，但任何保留的功能依赖都是完全部分。）

(The definition of 2NF involves full functional dependency of non-key attributes on keys but any functional dependecy that holds is either full or partial.)

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