A Comparison of Two Languages for Describing Stored Data,

摘要

Stored-Data Definition Languages have been developed to describe both the logical and the physical characteristics of stored-data. Two efforts,the Stored-Data Definition and Translation Task Group (SDDTTG) of the CODASYL Systems Committee and the Data Translation Project (DTP) at The University of Michigan,have produced Stored-Data Definition Lanugages. Although the translation methodologies are similar,the languages have different characteristics being influenced by their underlying models of data,reorganization,and translation capabilities. The language specified by SDDTTG (SDDL) has three main sections which describe the logical structure,the physical structure,and the correspondence between the two. The SDDL descriptions are based on the DIAM String model and,therefore are very adept at describing access paths. The SDDL also allows specification of validation information --checks that are performed on data instances during the translation process. Thus,the SDDL is reformatting oriented,low level,precise,and a rather complete language.