Relational Model

Declaring Keys in ODL

ODL is different from the other models studied so far in that the declaration and use of keys is optional. That is, in the E/R model, entity sets require keys to distinguish members of the entity set from one another. In the relational model, where relations are sets, all attributes

From ODL Designs to Relational Designs

As the E/R model is meant to be converted into a model such as the relational model when we put into operation the design as an actual database, ODL was initially meant to be used as the specification language for real, object-oriented DBMS's. Nevertheless ODL, like all object-

Nonatomic Attributes in Classes

Unluckily, even when a class properties are all attributes we may have some trouble converting the class to a relation. The reason is that attributes in ODL can have complicated types such as structures, sets, bags, or lists. On the other hand, a basic principle of the relational

Representing Set-Valued Attributes

Record structures are not the most complicated kind of attribute that can appear in ODL class definitions. Values can also be made using type constructors Set, Bag, List, Array, and Dictionary from Types in ODL. Each presents its own problems when migrating to the relational

The Object-Relational Model

The relational model and the object-oriented model typified by ODL are two important points in a spectrum of options that could underlie a DBMS. For an extended period, the relational model was dominant in the commercial DBMS world. Object-oriented DBMS's made limited

Nested Relations

Relations extended by point (1) of The Object-Relational Model are sometimes called "nested relations''. In the nested-relational model, we allow attributes of relations to have a type that is not atomic: particularly, a type can be a relation schema. Thus, there is a convenient,

From ODL Designs to Object-Relational Designs

In “From ODL Designs to Relational Designs” we learned how to convert designs in ODL into schemas of the relational model. Difficulties arose primarily because of the richer modeling constructs of ODL: nonatomic attribute types, relationships and methods. Some, but not all, of

Semistructured Data

The semistructured-data model plays an important role in database systems: 1. It serves as a model appropriate for integration of databases, that is, for describing the data included in two or more databases that include similar data with different schemas.

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

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