Two major flaws in the CODASYL DDL 1973 and proposed corrections

摘要

In a schema written in CODASYL DDL 1973, it is syntactically correct to describe an attribute of an entity either by declaring a data item in the record of the entity or by declaring a CODASYL set type in which the record, describing the entity, is a member record. This is considered a major flaw, because extension of a database or integration of two existing databases will then lead to either reprogramming or inconsistency, or both. Examples are described to illustrate situations which occur with a dynamic database. This flaw in the CODASYL DDL 1973 can easily be corrected by requiring that all attributes are represented as a data item in the (logical) schema.In the CODASYL DDL 1973, there are five places to optionally declare a record identifier, and four of these five places are not in the record but in the CODASYL set type. Declaring record identifiers therefore results in fairly complex and non-orthogonal declarations. This could be simplified by abandoning these five places and by introducing a record identifier clause in the record type entry. For integrity reasons, it is necessary to require that at least one record identifier is declared in every record type entry.The previous two corrections will make it possible to design a CODASYL set selection clause which is considerably simpler than the existing CODASYL set selection clause, still providing the same functional capabilities.The examples which were used to illustrate the first flaw are also described in schemas and programs using a DDL in which the two flaws are corrected.The conclusion is: the corrected DDL is functionally equivalent, yet offers more data independence, is simpler and more orthogonal. Or, said in other words this article concentrates on the major issue of the “great debate” between the network (CODASYL) and relational data base approach. At the end of studying this article the reader is supposed to comprehend the major portion of the contents of the “great debate”.