What Do Foreign Keys Actually Mean?

Foreign keys form a major structuring construct in relational databases and in standard files. In reverse engineering processes, they have long been interpreted as the implementation of many-to-one relationship types, not only in relational databases but also in legacy hierarchical and network databases. Besides the standard version of foreign key, according to which a set of columns (fields) in a table (file) is used to designate rows (records) in another table, a careful analysis of existing databases puts into light a surprisingly large variety of non standard forms of foreign keys. Most of them are quite correct, and perfectly fitted to the requirements the developer had in mind. However, their conceptual interpretation can prove more difficult to formalize than the standard forms. This paper classifies, analyzes and interprets some outstanding variants of foreign keys that were observed in operational files and databases during more than 20 years of database reverse engineering. So, it is aimed to contribute to a better understanding of database schemas.