Schema

Decomposition into Fourth Normal Form

The 4NF decomposition algorithm is quite similar to the BCNF decomposition algorithm. We find a 4NF violation, say A1A2 . . . An → B1B2 . . . Bm , where {A1, A2, . . . , An} is not a superkey. Note this MVD could be a true MVD, or it could be derived from the equivalent FD A1A2 . . . An

Relationships Among Normal Forms

As we have pointed out, 4NF implies BCNF, which in turn implies 3NF. Therefore, the sets of relation schemas (including dependencies) satisfying the three normal forms are related as in the following figure. That is, if a relation with certain dependencies is in 4NF, it is also in

Representing Other Type Constructors

Above and beyond record structures and sets, an ODL class definition could use Bag, List, Array, or Dictionary to construct values. To represent a bag (multiset), in which a single object can be a member of the bag n times, we cannot simply introduce into a relation n the

Representing ODL Relationships

Generally, an ODL class definition will include relationships to other ODL classes. As in the E/R model, we can create for each relationship a new relation that connects the keys of the two related classes. However, in ODL, relationships come in inverse pairs, and we must create

What If There Is No Key

Since keys are optional in ODL, we may face a situation where the attributes available to us cannot serve to represent objects of a class C uniquely. That situation can be a problem if the class C participates in one or more relationships.

References

The fact that movies like Star Wars will appear in various relations that are values of the movies attribute in the nested relation Stars is a cause of redundancy. In effect, the schema of Nested Relations Example (a) has the nested-relation analog of not being in BCNF. On the

XML and Its Data Model

XML (Extensible Markup Language) is a tag-based notation for "marking" documents, much similar to the familiar HTML or less familiar SGML. A document is nothing more nor less than a file of characters. However, while HMTL's tags talk about the presentation of the information

Document Type Definitions

In order for a computer to process XML documents automatically, there needs to be something like a schema for the documents. That is, we need to be told what tags can appear in a collection of documents and how tags can be nested. The description of the schema is given by a

Relational Algebra

This section begins a study of database programming, that is, how the user can ask queries of the database and can change the contents of the database. Our focus is on the relational model, and especially on a notation for describing queries about the content of relations called

Set Operations on Relations

The three most common operations on sets are union, intersection, and difference. We assume the reader is familiar with these operations, which are described as follows on arbitrary sets R and S:

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