Relational Databases

You can contact the author of this section, Barney Finucane, if you have any comments, observations or user experiences to add. Last updated January 28th, 2010.

Relational databases have been used by businesses for decades and they have steadily developed the features required for large-scale implementation, including scalability (in terms of user count and database size). The idea of relational databases was first suggested by Edgar Codd in 1969, and by the nineties, they had largely replaced hierarchical and network databases for business purposes. But the original definition is excruciatingly abstract, and no Codd-standard “true” relational database has ever been commercially released, although there are academic implementations. Textbook discussions of relational databases descend into pure math almost immediately. For practical purposes, relational databases are databases that can be queried using SQL, but purists point out that the current ISO standard definition of SQL does not mention the relational model and ISO SQL deviates from the relational model in several ways.

Roughly speaking, a relational database is a set of data connected by relations. To take a simple example, an invoicing system could have one table with a list of invoices and another with a list of customers. Each table has one record (referred to as a tuple) per line, and defines a relation – for example, the invoice number is related to the date the invoice was issued. In fact, relational tables are often referred to as “base relations”.

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