A relational database is a database that groups data using common attributes found in the data set. The resulting "clumps" of organized data is much easier for people to understand.

For example, a data set containing all the real estate transactions in a town can be grouped by the year the transaction occurred; or it can be grouped by the sale price of the transaction; or it can be grouped by the buyer's last name; and so on.

Such a grouping uses the relational model (a technical term for this schema). Hence such a database is called a "relational database."

The software used to do this grouping is called a relational database management system. The term "Relational database" often refers to this type of software.

Relational databases have become the overwhelming choice for the storage of the tabular information that supports the world economy: including financial records, manufacturing and logistical information, personnel data and much more.

Contents

Strictly, a relational database is a collection of relations (frequently called tables). Other items are frequently considered part of the database, as they help to organize and structure the data, in addition to forcing the database to conform to a set of requirements.

Terminology

The term relational database was originally defined and coined by Edgar Codd at IBM Almaden Research Center in 1970.

Relational database theory uses a different set of mathematical-based terms, which are equivalent, or roughly equivalent, to SQL database terminology. The table below summarizes some of the most important relational database terms and their SQL database equivalents.

Relations or tables

main: Table (database)

A relation is defined as a set of tuples that have the same attributes. A tuple usually represents an object and information about that object. Objects are typically physical objects or concepts. A relation is usually described as a table, which is organized into rows and columns. All the data referenced by an attribute are in the same domain and conform to the same constraints.

The relational model specifies that the tuples of a relation have no specific order and that the tuples, in turn, impose no order on the attributes. Applications access data by specifying queries, which use operations such as select to identify tuples, project to identify attributes, and join to combine relations. Relations can be modified using the insert, delete, and update operators. New tuples can supply explicit values or be derived from a query. Similarly, queries identify tuples for updating or deleting.

Base and derived relations

main: View (database)

In a relational database, all data are stored and accessed via relations. Relations that store data are called "base relations", and in implementations are called "tables". Other relations do not store data, but are computed by applying relational operations to other relations. These relations are sometimes called "derived relations". In implementations these are called "views" or "queries". Derived relations are convenient in that though they may grab information from several relations, they act as a single relation. Also, derived relations can be used as an abstraction layer.