Chapter 5 Legacy Databases

In this chapter, we briefly zoom into legacy database technologies and their logical data models. Although these are outdated and semantically inferior to modern day database technologies, the reasons for reviewing them are threefold. First, many firms still struggle with legacy databases due to historical implementations and limited IT budgets. Hence, knowing the basic characteristics thereof is essential to the maintenance of the corresponding database applications and the potential migration to modern DBMSs. Second, understanding the basics of these legacy models will contribute to better understanding the semantical richness of newer database technologies. Finally, the concept of procedural DML and navigational access originally introduced by these legacy models has also been adopted by more recent databases such as OODBMSs.
In this chapter, we cover both the hierarchical and CODASYL data model. Throughout the discussion, we extensively discuss the expressive power and limitations of both models.