Note on the structure of joins

摘要

When SQL is used to formulate queries for a relational database, many conditions in the WHERE clause appear to be very predictable. These are the so-called join conditions which indicate how the tables of a database are related. It seems that a system should be able to generate these conditions to a great extent automatically from the knowledge of the database structure. To this end the notion of the structure of a join is introduced and mathematically described as a graph morphism. It turns out to be a generalization of the notion of a natural join. It is claimed that this approach is theoretically elegant and provides in practice a good basis for the development of query generators.