普通表格-第4普通表格 [英] Normal Form - 4th Normal Form

问题描述

我有需要输入到第三和第四范式的数据。

I have this data that I need to put into 3rd and 4th Normal Form.

我了解范式的基本知识，但是对第三和第四范本感到困惑

I understand the basics of normal form but I'm confused with 3rd and 4th normal form and I've checked online but still don't understand.

我正在创建一个系统正在使用的数据库。

I'm creating a database which is being used by my system.

推荐答案

在BCNF，3NF和较低NF中也存在4NF的关系。

A relation in 4NF is also in BCNF, 3NF and the lower NFs.

来自此答案：

超键是一组唯一标识行的列。

A superkey is a set of columns that uniquely identifies rows.

候选键是一组唯一标识行的列，而
不包含唯一的
标识行的列的较小（适当）子集。

A candidate key is a set of columns that uniquely identifies rows and that contains no smaller ("proper") subset of columns that uniquely identifies rows.

因此，
候选键是一个不包含更小的超级键的超级键。

So a candidate key is a superkey that contains no smaller superkey.

A 多值依赖项（MVD）X->>当表可以表示为两个表的联接时，Y将其保存在具有属性集R的表中（即，该表具有该MVD），这是它在属性集X U Y和X U（R-Y）上的投影。 （因为MVD是二进制加入依赖项（JD））。

A multi-valued dependecy (MVD) X ->> Y holds in a table (ie a table has that MVD) with attribute set R when the table can be expressed as the join of the two tables that are its projections on attribute sets X U Y and X U (R - Y). (Because an MVD is a binary join dependency (JD)).

来自维基百科：

p一个琐碎的多值依赖项X-> Y是
之一，Y是X的子集，或者X和Y共同构成关系的整个
属性。

A trivial multivalued dependency X ->> Y is one where either Y is a subset of X, or X and Y together form the whole set of attributes of the relation.

表位于对于且仅当对于其每个非平凡的
多值依赖项X->> 4NF Y，X是一个超键-
是，X是候选键或其超集。

A Table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X ->> Y, X is a superkey—that is, X is either a candidate key or a superset thereof.

每个表都有一个 predicate ，即由表的属性名称参数化的语句模板，其中表中的行是通过用行中相应的属性值替换谓词中的属性名称来构成真实语句的行。 MVD X-> Y的谓词 P 可以表述为以下形式时保持在表中：


对于不在XUY中的属性的某些值， P


并为不在XU（R-Y）， P

Every table has a predicate, ie a statement template parameterized by the table's attribute names, where the rows that go in the table are the ones that make a true statement by replacing attribute names in the predicate by the corresponding attribute values in the row. The MVD X ->> Y holds in a table when its predicate P can be rephrased as:
         FOR SOME values for the attributes not in X U Y, P
     AND FOR SOME values for the attributes not in X U (R - Y), P

中的属性设置一些值您表的含义是CK，超键和MVD的指南。要将BCNF标准化为4NF：如果是非平凡的MVD，则X>>。 Y不在超级键中，然后用X U Y和X U（R-Y）上的投影替换表。在新表上重复。

So let your table's meaning be your guide to CKs, superkeys and MVDs. To normalize from BCNF to 4NF: If a non-trivial MVD X ->> Y not out of a superkey holds then replace the table by its projections on X U Y and X U (R - Y); repeat on the new tables.

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