Database

A database is a group of related files. This collection is usually organized to facilitate efficient and accurate inquiry and update. A database management system (DBMS) is a software package that is used to organize and maintain a database.

Usually when we use the word "file" we mean traditional or conventional files. Sometimes we call them "flat files." With these traditional, flat files each file is a single, recognizable, distinct entity on your hard disk. These are the kind of files that you can see cataloged in your directory.

Commonly, these days, when we use the word "database" we are not talking about a collection of this kind of file; rather we would usually be understood to be talking about a database management system. And, commonly, people who work in a DBMS environment speak in terms of "tables" rather than "files."

DBMS software allows data and file relationships to be created, maintained, and reported. A DBMS offers a number of advantages over file-oriented systems including reduced data duplication, easier reporting, improved security, and more rapid development of new applications.

The DBMS may or may not store a table as an individual, distinct disk file. The software may choose to store more than one table in a single disk file. Or it may choose to store one table across several distinct disk files, or even spread it across multiple hard disks. The details of physical storage of the data is not important to the end user who only is concerned about the logical tables, not physical disk files.

In a hierarchical database the data is organized in a tree structure. Each parent record may have multiple child records, but any child may only have one parent. The parent-child relationships are established when the database is first generated, which makes later modification more difficult.

A network database is similar to a hierarchical database except that a child record (called a "member") may have more than one parent (called an "owner"). Like in a hierarchical database, the parent-child relationships must be defined before the database is put into use, and the addition or modification of fields requires the relationships to be redefined.

In a relational database the data is organized in tables that are called "relations." Tables are usually depicted as a grid of rows ("tuples") and columns ("attributes"). Each row is a record; each column is a field. With a relational database links between tables can be established at any time provided the tables have a field in common. This allows for a great amount of flexibility.