October 2013 Archive

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.

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,


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

Object-Oriented Versus Object-Relational

The object-oriented data model, as typified by ODL, and the object-relational model discussed here, are outstandingly similar. Some of the most important points of comparison follow.

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.

Semistructured Data Representation

A database of semistructured data is a collection of nodes. Each node is either a leaf or interior. Leaf nodes have associated data; the type of this data can be any atomic type, such as numbers and strings. Interior nodes have one or more arcs out. Each arc has a label, which shows

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