Shulou(Shulou.com)05/31 Report--

Today, I will talk to you about what is multi-valued dependency in the database. Many people may not know much about it. In order to make you understand better, the editor summarized the following content for you. I hope you can gain something according to this article.

"Multi-valued dependency" is an assertion that two attributes or sets of properties are independent of each other. This situation is a generalized form of the concept of functional dependencies, which means that each functional dependency contains a corresponding multi-valued dependency. However, some cases involving the independence of attribute sets cannot be interpreted as functional dependencies. In this article, we will work with you to find out the causes of multi-valued dependencies and see how they can be used in database schema design.

The independence of attributes and their redundancy occasionally encounter this situation. We design a relational schema and find that it belongs to BCN F, but the relationship still has some redundancy that has nothing to do with function dependencies. The most common reason for redundancy in BCNF schemas is the independence of two or more multi-valued attributes of a class when we convert ODL schemas to relational schemas.

"Multi-valued dependency" is a statement about a relation R, which means that if the value of a set of attributes of R is determined, the values of some other specific attributes are independent of the values of all other attributes of the relationship. More specifically, if we define the tuple of R to take a specific value on each attribute belonging to A, and the result is that the set of attribute values belonging to B has nothing to do with the set of attribute values that neither belong to A nor B but belong to R, then we say that multi-valued dependence A 1A 2; A n; B1B 2; Bm holds in relation R. More specifically, for each pair of tuples t and u in relation R that have the same values on all attributes of A, we can find a tuple v in R that satisfies:

1. The values are consistent with t and u on A.

two。 The value is the same as t on B, and

3. And u have the same values on all attributes of R except An and B. Then we call this multi-valued dependence valid.

Note that t and u are interchangeable in the above rule, which means that there is a fourth tuple w, which is consistent with u on B and t on other attributes. As a result, for any fixed value of A, the related values of B and other attributes appear in all possible combinations in different tuples.

In general, we might assume that the attributes in An and B of multi-valued dependencies (left and right) are separate. However, like functional dependencies, some properties in An are allowed to appear on the right if we wish. Also note that in functional dependencies we start with only one attribute on the right, and allow the right side to be a simplified form of the attribute set. At this point, unlike functional dependencies, we must directly consider the case where the right side is a property set.

After reading the above, do you have any further understanding of what multivalued dependencies are in the database? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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