Relational Databases and SQL

A relational database is a database that utilizes the relational model of data. The relational model, invented by Edgar F. Codd in 1970, organizes data as relations, or sets of tuples. Each tuple consists of a series of attributes, which generally describe the tuple. For example, we could imagine a customer relation, where each tuple represents a customer. Each tuple would then have attributes describing a single customer, giving information such as first name, last name, and age, perhaps in the format (John, Smith, 27). One or more of the attributes is used to uniquely identify a tuple in a relation and is called the relational key. The relational model then allows logical operations to be performed between relations.

In a relational database, relations are usually implemented as tables, as in an Excel spreadsheet. Each row of the table is a tuple, and the attributes are represented as columns of the table. While not technically required, most tables...