数据库范式5nf_第四范式（4NF）数据库管理系统
数据库范式5nf
Fourth normal form (4NF) is a normal form ud in , in which there are no non-trivial multivalued dependencies except a Fourth normal form (4NF)
candidate key. After Boyce–Codd normal form (BCNF), 4NF is the next level of normalization. Although the cond, third, and Boyce–Codd normal forms operate with functional dependencies, 4NF is operated with a more universal type of dependency known as a multivalued dependency.
第四范式(4NF)是使⽤的 ，其中没有除候选键之外的⾮平凡多值依赖项。 在Boyce-Codd范式(BCNF)之后，4NF是下⼀级别的规范化。第四范式(4NF)
尽管第⼆，第三和Boyce-Codd范式具有功能依赖性，但4NF却具有更为通⽤的依赖性类型，即多值依赖性。
A table or relation is in 4NF if and only if, for each attribute of its non-trivial multivalued dependencies P→Q, P is a super key—that means, P is either a candidate key or a supert thence.
当且仅当表或关系对于其⾮平凡多值依赖项P→Q的每个属性，P是⼀个超级键，也就是说，P是⼀个候选键或⼀个⽗集，才在4NF中。Properties:
特性：
A relation R with attributes, its values and tuples is in 4NF if and only if the following conditions are satisfied.
当且仅当满⾜以下条件时，带有属性，其值和元组的关系R才为4NF。
1. The relation or table should be in the Boyce-Codd Normal Form (BCNF).
关系或表格应采⽤Boyce-Codd范式(BCNF)。
2. The relation or table should not have any Multivalued Dependency.
关系或表不应具有任何多值依赖关系。
Fourth Normal Form (4NF) A table which consists of a multivalued dependency breaches the normalization attainment of Fourth Normal Form (4NF) becau it generates unneeded redundancie
s and can contribute to incompatible data which will not be inconsistency. To take this forward and up towards 4NF, it is important to divide this data and information into two tables or relations.
第四范式(4NF)的归⼀化程度，因为它产⽣了不必要的冗余，并且可能导致不兼容的数据，并且不会造成由多值依赖关系组成的表违反了第四范式(4NF)
不⼀致。 为了朝着4NF迈进，将这些数据和信息分成两个表或关系很重要。
Multivalued Dependency
多值依赖
Multivalued dependency takes place when there are one or more rows in a table or relation which indicates the existence of one or more other rows in the same table or relation. In another way, if we obrve, two attributes (or columns) in a table or relation are not dependent on each other, but both of them depend on a third attribute. A multivalued dependency always needs a minimum of three attributes becau it contains at least two attributes that are dependent on a third.酸根离子是什么
当表或关系中存在⼀个或多个⾏时，即表⽰同⼀表或关系中存在⼀个或多个其他⾏，则发⽣多值依赖。 ⽤另⼀种⽅式，如果我们观察到，表或关系中的两个属性(或列)彼此不依赖，但是它们两者都依
赖于第三个属性。 多值依赖项始终⾄少需要三个属性，因为它包含⾄少两个依赖于第三个属性。
For a Functional dependency in which P holds Q (P -> Q), if for a single value of P, multiple values of Q exists, then the table or relation may have a multi-valued dependency. The table or relation should contain minimally three attributes and Q and S should not be dependent for P ->> Q multivalued dependency.
闭合性跌打损伤是什么意思对于其中P保持Q(P-> Q)的功能依赖关系，如果对于P的单个值，存在Q的多个值，则表或关系可能具有多值依赖关系。 该表或关系应⾄少包含三个属性，并且Q和S不应依赖于P->> Q多值依赖关系。
If the following points are satisfied, then a table or relation has a multivalued dependency,
如果满⾜以下⼏点，则表或关系具有多值依赖关系，
票的英文1. In a table, for a Functional Dependency P which holds Q (P → Q), if for a single value of P, multiple value of Q exists,
then the table may have multi-valued dependency.
在⼀个表中，对于具有Q(P→Q)的功能依赖性P，如果对于P的单个值，存在Q的多个值，则该表可能具有多值依赖性。
2. Also, the condition is that a table should have at-least three columns for it to have a multivalued dependency.
同样，条件是⼀个表应⾄少具有三列，以使其具有多值依赖关系。
3. And, for a relation R(P, Q, S), if there is a multi-valued dependency between P and Q, then Q and S should not be
双人行dependent on each other.
并且，对于关系R(P，Q，S)，如果在P和Q之间存在多值依赖关系，则Q和S不应相互依赖。
Example:
例：
Consider a databa table which contain student Name, Hobby and subject,
考虑⼀个包含学⽣姓名，爱好和科⽬的数据库表，
Name Hobby Subject
Arvind Cricket Maths
Sameer Football Science
Rajeev Chess Social science
Simran Badminton English
Rajat Singing Hindi
名称爱好学科
阿⽂德蟋蟀数学
沙美尔⾜球科学
拉杰夫棋社会科学
上浆
西姆兰⽻⽑球英语
拉贾特唱歌印地语
In the above table, there is no connection between the columns of Hobby and Subject. They are both independent of each other.
在上表中，“爱好”和“主题”列之间没有任何联系。 它们彼此独⽴。
So there is a multivalued dependency, which assists towards unneeded repetition and redundancy of data and other anomalies as well.
因此，存在⼀个多值依赖关系，它也有助于不必要的重复和数据冗余以及其他异常。
fourth normal form condition and its properties, we can divide the table into To make the above relation satisfy with the fourth normal form
two tables, which contain Name and Subject in one table and Name and Hobby in another table.
为了使上述关系满⾜第四范式
第四范式条件及其属性，我们可以将表分为两个表，⼀个表中包含Name和Subject，另⼀表中包含Name和Hobby。
Name Hobby
男服务员
Arvind Cricket
Sameer Football
Rajeev Chess
电信itvSimran Badminton
Rajat Singing
名称爱好
阿⽂德蟋蟀
沙美尔⾜球
拉杰夫棋
西姆兰⽻⽑球
拉贾特唱歌
Name Subject
Arvind Maths
Sameer Science
Rajeev Social science
Simran English
Rajat Hindi
名称学科
阿⽂德数学
沙美尔科学
拉杰夫社会科学做人做事的基本原则
西姆兰英语
拉贾特印地语
Now, this relation satisfied the conditions of the fourth normal form.
现在，该关系满⾜第四范式的条件。
A table can also contain a functional dependency along with multi-valued dependency. In that ca, the functionally dependent columns are transferred in a parate table and the multi-valued dependent columns are transferred to parate tables.
表还可以包含功能依赖项以及多值依赖项。 在这种情况下，功能相关的列将转移到单独的表中，多值相关的列将转移到单独的表中。
数据库范式5nf
