are linked into a network in which any given link between two items exists because it satisfies some condition on the attributes of those items, for example, they share an attribute.
It is more general than the hierarchic approach in the sense that a node can have any number of immediate superiors.
It is also equivalent to the relational approach in descriptive power.
The whole field of data base structures is still very much in a state of flux.
The advantages and disadvantages of each approach are discussed very thoroughly in Date, who also gives excellent annotated citations to the current literature.
There is also a recent Computing Survey[ll] which reviews the current state of the art.
There have been some very early proponents of the relational approach in IR, as early as 1967 Maron and Levien discussed the design and implementation of an IR system via relations, be it binary ones.
Also Prywes and Smith in their review chapter in the Annual Review of Information Science and Technology more recently recommended the DBTG proposals as ways of implementing IR systems.
Lurking in the background of any discussion of file structures nowadays is always the question whether data base technology will overtake all.
Thus it may be that any application in the field of library automation and information retrieval will be implemented through the use of some appropriate data base package.
This is certainly a possibility but not likely to happen in the near future.
There are several reasons.
One is that data base systems are general purpose systems whereas automated library and retrieval systems are special purpose.
Normally one pays a price for generality and in this case it is still too great.
Secondly, there now is a considerable investment in providing special purpose systems (for example, MARC) and this is not written off very easily.
Nevertheless a trend towards increasing use of data-base technology exists and is well illustrated by the increased prominence given to it in the Annual Review of Information Science and Technology.
A language for describing file structures
Like all subjects in computer science the terminology of file structures has evolved higgledy-piggledy without much concern for consistency, ambiguity, or whether it was possible to make the kind of distinctions that were important.
It was only much later that the need for a well-defined, unambiguous language to describe file structures became apparent.
In particular, there arose a need to communicate ideas about file structures without getting bogged down by hardware considerations.