The Logical Data Model: A New Approach to Database Logic.
Abstract
We propose a mathematical framework for unifying and generalizing the principal data models, i.e., the relational, hierarchical and network models. Until recently most theoretical work on databases has focused on the relational model, mainly due to its elegance and mathematical simplicity compared to the other models. Some of this work has pointed out various disadvantages of the relational model, among them its lack of semantics and the fact that it forces the data to have a flat structure that the real data does not always have. The Logical Data Model (LDM) combines the advantages of the relational, network and hierarchical approaches. It models database schemas as directed graphs, in which the leaves correspond to the attributes, and the internal nodes to connections between the data. Instances of LDM schemas consist of r-values, which constitute the data space, and l-values, which constitute the address space. We are thus able to deal with instances of cyclic structures, but still get a first-order theory. We define a logic on LDM schemas in which integrity constraints can be specified, and use it to define a logical, i.e., non-procedural, query language that is analogous to Codd's relational calculus. We also describe an algebraic, i.e., procedural, query language and prove that the two languages are equivalent. These languages have a novel feature: not only can they access a non-flat data structure, e.g. a hierarchy, but the answers they produce do not have to be flat either. Thus, the language really does have the ability to restructure data and not only to retrieve it, and can therefore be used both as a query language and for defining views.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 1985
- Accession Number
- ADA323935
Entities
People
- Gabriel M. Kuper
Organizations
- Stanford University