Summary

A database contains a collection of related information but differs in a fundamental way from other methods of storing data. The database contains not only the data, but also a data model that defines each data item and specifies its meaning with respect to the other items and to the real world.

More than one user can access and modify a database at the same time. Each user has a different view of the contents of a database, and each user's access to those contents can be restricted in several ways.

A relational database consists of tables, and the tables consist of columns and rows. The relational model supports three fundamental operations on tables: selections, projections, and joins.

An object-relational database extends the features of a relational database. You can define new data types to store and manage audio, video, large text documents, and so forth. You can define complex types that combine one or more existing data types to provide greater flexibility in how you organize your data in columns and tables. You can define types and tables that inherit the properties of other database objects and add new properties that are specific to the object that you define.

To manipulate and query a database, use SQL. IBM pioneered SQL and ANSI standardized it. HCL® OneDB® extensions that you can use to your advantage add to the ANSI-defined language. HCL OneDB tools also make it possible to maintain strict compliance with ANSI standards.

Two layers of software mediate all your work with databases. The bottom layer is always a database server that executes SQL statements and manages the data on disk and in computer memory. The top layer is one of many applications, some from IBM and some written by you, by other vendors, or your colleagues. Middleware is the component that links the database server to the application, and is provided by the database vendor to bind the client programs with the database server. HCL OneDB Stored Procedure Language (SPL) is an example of such a tool.