relational database

Database in which all data are represented in tabular form. The description of a particular entity is provided by the set of its attribute values, stored as one row or record of the table, called a tuple. Similar items from different records can appear in a table column. The relational approach supports queries that involve several tables by providing automatic links across tables. Payroll data, for example, can be stored in one table and personnel benefits data in another; complete information on an employee can be obtained by joining the tables on employee identification number. In more powerful relational data models, entries can be programs, text, unstructured data in the form of binary large objects (BLOBs), or any other format the user requires. The relational approach is currently the most popular model for database management system. See also object-oriented programming.

A database that maintains a set of separate, related files (tables), but combines data elements from the files for queries and reports when required. The concept was developed in 1970 by Edgar Codd, whose objective was to accommodate a user's ad hoc request for selected data. Most every business database management system (DBMS), including Oracle, DB2, SQL Server, MySQL, etc., is a relational DBMS (RDBMS) (see DBMS).

Hierarchical, Network and Object Databases
In non-relational "hierarchical" and "network" databases, records in one file contain embedded pointers to the locations of records in another, such as customers to orders and vendors to purchases. These are fixed links set up ahead of time to speed up daily processing. Another type of non-relational database is the "object database," which stores data consistent with their object model (see object database).

Comparing and Joining
Routine queries to a relational database often require data from more than one file. For example, to obtain the names of customers who purchased a particular product, data must be extracted from both the customer and order files. A relational DBMS has the flexibility to "join" two or more files by comparing key fields such as account number and name and generating a new file from the records that meet the matching criteria (see join).

Indexes Are Used
In practice, a pure relational query can be very slow. In order to speed up the process, indexes are built and maintained on the key fields used for matching. Sometimes, indexes are created "on the fly" when the data are requested.

     Relational Terms      Common Terms

     Table or Relation     File
     Row or Tuple          Record
     Attribute or Field    Field

(database)

relational database - (RDBMS - relational database management system) A database based on the relational model developed by E.F. Codd. A relational database allows the definition of data structures, storage and retrieval operations and integrity constraints. In such a database the data and relations between them are organised in tables. A table is a collection of rows or records and each row in a table contains the same fields. Certain fields may be designated as keys, which means that searches for specific values of that field will use indexing to speed them up.

Where fields in two different tables take values from the same set, a join operation can be performed to select related records in the two tables by matching values in those fields. Often, but not always, the fields will have the same name in both tables. For example, an "orders" table might contain (customer_id, product_code) pairs and a "products" table might contain (product_code, price) pairs so to calculate a given customer's bill you would sum the prices of all products ordered by that customer by joining on the product-code fields of the two tables. This can be extended to joining multiple tables on multiple fields. Because these relationships are only specified at retreival time, relational databases are classed as dynamic database management system.

The first commercial RDBMS was the Multics Relational Data Store, first sold in 1978.

INGRES, Oracle, Sybase, Inc., Microsoft Access, and Microsoft SQL Server are well-known database products and companies. Others include PostgreSQL, SQL/DS, and RDB.

["Managing Data Bases, Four Critical Factors" Michael M. Gorman, QED Information Sciences, Inc.].

["An Introduction To Database Systems" (6th ed) C. J. Date, Addison Wesley (an excellent source of detailed info)].

["An End-User's Guide to Data Base" James Martin, Prentice Hall (excellent place to begin learning about DBMS)].