Databases

Overview

A database-management system (DBMS) is a collection of interrelated data and a set of programs to access those data. The collection of data is usually referred to as a database. Data is usually abstracted across the following levels:

1. Physical Level. Describes how data is stored. Usually involves complex low-level data structures in detail.
2. Logical Level. Describes what data is stored and the relationships that exist among those data.
3. View Level. Describes a selected portion of the database.

The separation of the physical and logical level is referred to as physical data independence.

Modes

DBMSs are broadly categorized as online transaction processing (OLTP) and online analytical processing (OLAP). Roughly speaking, the former focuses on processing database transactions whereas the latter focuses on analyzing aggregated data.

Language

A DBMS provides a data definition language (DDL) to specify schemas and a data manipulation language (DML) to express database queries and updates. Usually the DDL and DML form parts of a single database language (e.g. SQL).

A data control language (DCL) refers to the subset of a DDL that concerns itself with authorization.

A data query language (DQL) refers to the subset of a DML that concerns itself with actively querying information from the database. A transaction control language (TCL) is another subset of a DML concerning itself with the management of transactions.