数据库范式5nf-第四范式（4NF）数据库管理系统.pdf

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) Fourth normal form (4NF) is a normal form used in , in which there are no non-trivial multivalued dependencies except a candidate key. After Boyce–Codd normal form (BCNF), 4NF is the next level of normalization. Although the second, 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) 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。 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 superset 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: Properties: 特性： 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。 A table which consists of a multivalued dependency breaches the normalization attainment of Fourth Normal Form (4NF) Fourth Normal Form (4NF) because it generates unneeded redundancies 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. 由多值依赖关系组成的表违反了第四范式(4NF) 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 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 Multivalued Dependency 多值依赖 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 observe, two attributes (or columns)