During the last three or four years several investigators have been exploring “semantic models ” for formatted databases. The intent is to capture (in a more or less formal way) more of the meaning of the data so that database design can become more systematic and the database system itself can behave more intelligently. Two major thrusts are clear: (I) the search for meaningful units that are as small as possible--atomic semantics; (2) the search for meaningful units that are larger than the usual n-ary relation-molecular semantics. In this paper we propose extensions to the relational model to support certain atomic and molecular semantics. These extensions represent a synthesis of many ideas from the published work in semantic modeling plus the introduction of new rules for insertion, update, and deletion, as well as new algebraic operators.

We examine the problem of type evolution in an objectoriented database environment. Type definitions are persistent objects in the database and as such may be modified and shared. The effeets of changing a type extend to objects of the type and to programs that use objeets of the type. We propese a solution to the problem through an extension of the semantic data model. A ehange in the interfaee defined by a type may result in errors when programs use new or old objects of the type. Through the use of an abstraction of the type over time, timestamping and error handling mechanisms provide support for the type designer in creating compatible versions of the type. The mechanisms are incorporated Into the behavior defined by the type and are inherited via the type-lattice. 1.

Hypertext systems are being used in many applications because of their flexible structure and the great browsing freedom they give to diverse communities of users. However, this same freedom and flexibility is the cause of one of its main problem: the "lost in hyperspace" problem. One reason for the complexity of hypertext databases is the large number of nodes and links that compose them. To simplify this structure we propose that nodes and links be clustered forming more abstract structures. An abstraction is the concealment of all but relevant properties from an object or concept. One type of abstraction is called an aggregate. An aggregate is a set of distinct concepts that taken together form a more abstract concept. For example, two legs, a trunk, two arms and a head can be aggregate together in a single higher level object called a "body." In this paper we will study the hypertext structure, i.e., the way nodes are linked to each other in order to find aggregates in hypertext da...

A new normal form for relational databases, called domain-key normal form (DK/NF), is defined. Also, formal definitions of insertion anomaly and deletion anomaly are presented. It is shown that a schema is in DK/NF if and only if it has no insertion or deletion anomalies. Unlike previously defined normal forms, DK/NF is not defined in terms of traditional dependencies (functional, multivalued, or join). Instead, it is defined in terms of the more primitive concepts of domain and key, along with the general concept of a “constraint. ” We also consider how the definitions of traditional normal forms might be modified by taking into consideration, for the first time, the combinatorial consequences of bounded domain sizes. It is shown that after this modification, these traditional normal forms are all implied by DK/NF. In particular, if all domains are infinite, then these traditional normal forms are all implied by DK/NF.

Record structures are generally efficient, familiar, and easy to use for most current data processing applications. But they are not complete in their ability to represent information, nor are they fully self-describing.

The data model upon which most of today's commercial database management systems are based has shown to be insufficient for geographic information systems (GISs). The recently promoted object-oriented model provides some useful tools for data abstraction and data structuring, which augment the conventional tools and overcomes some deficiencies inherent to the traditional relational model. In particular, the concepts of complex objects with pertinent operations are more powerful modeling methods than the currently popular structure of relational tables and relational algebra. This survey article presents the concepts of object-oriented modeling applied to geographic data and demonstrates their impact on future GISs.

Current geographic information systems (GISs) have been designed for querying and maintaining static databases representing static phenomena and give little support to those users who wish to represent dynamic information or incorporate temporality into their studies. In order to integrate phenomena that change over space and time in GISs, a better understanding of the underlying components of change and how people reason about change is needed. This paper focuses on a qualitative representation of change. It offers a classification of change based on object identity and the set of operations that either preserve or change identity. These operations can be applied to single or composite objects and combined to express the semantics of sequences of change. An iconic, visual language is developed to represent the various types of change and applied to examples to illustrate the application of this language. Such a formalization of the basic components of change lays the foundation for a new generation of formal models that captures the semantics of change and leads to improved interoperability between GISs and process models or simulation software.

We investigate the complexity of query processing in the logical data model (LDM). We use two measures: data complexity, which is complexity with respect to the size of the data, and expression complexity, which is complexity with respect to the size of the expressions denoting the queries. Our investigation shows that while the operations of product and union are essentially first-order operations, the power set operation is inherently a higher-order operation and is exponentially expensive. We define a hierarchy of queries based on the depth of nesting of power set operations and show that this hierarchy corresponds to a natural hierarchy of Turing machines that run in multiply exponential time.

The paper addresses the problem of the automatic construction of a hypertext to be used for Information Retrieval (IR) purposes. The starting point is modelling a set of raw IR data by means of a conceptual schema. The conceptual schema produced for a specific IR application domain provides the user with a frame of reference in the query formulation process. This schema can be implemented by means of a hypertext tool using a set of automatic techniques for the identification of nodes and links. The hypertext tool can be used for querying processing, as an automatic tool of query expansion, and for browsing, enabling the user to query and move through the IR data using different levels of abstraction. This paper describes the modelling and design process and concentrates on the set of automatic techniques to be used for the automatic construction of the IR hypertext. 1 Introduction Information Retrieval (IR) systems are typically implemented as stand-alone systems for efficient storage...

Incorporating abstraction methods, such as aggregation and association, into information system design methodologies has improved our ability to model the real world. The semantically-higher level objects that result from these abstractions are referred to as composite objects. These objects play an important role in spatio-temporal knowledge representation and query formulation, although little has been done so far on formalizing operations involving these types of objects. In this investigation, the semantics associated with composite objects are explored as is the role of object identity for composite objects. Object identity refers to that trait which distinguishes an object from all others. The different semantics associated with creating composite objects and adding parts to composites are discussed and a set of basic identity-based change operations for composites, including separation and elimination operations, are described. Formalizing the operations relating to composite objects aids in improving current spatial data models and leads to advances in spatial-temporal query languages.