A database consists of an organized collection of data The term data refers to groups of information that represent the qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which information and for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems A Database Management System is a set of computer programs that controls the creation, maintenance, and the use of the database with computer as a platform or of an organization and its end users. It allows organizations to place control of organization-wide database development in the hands of database administrators (DBAs) and other specialists, which store database contents, allowing data creation and maintenance, and search and other access.

Architecture

Database architecture consists of three levels, external, conceptual and internal. Clearly separating the three levels was a major feature of the relational database model The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by E.F. Codd that dominates 21st century databases.^[1]

The external level defines how users understand the organization of the data. A single database can have any number of views at the external level. The internal level defines how the data is physically stored and processed by the computing system. Internal architecture is concerned with cost, performance, scalability and other operational matters. The conceptual is a level of indirection between internal and external. It provides a common view of the database that is uncomplicated by details of how the data is stored or managed, and that can unify the various external views into a coherent whole.^[1]

Database management systems

A database management system (DBMS) consists of software Computer software, or just software, is the collection of computer programs and related data that provide the instructions telling a computer what to do. The term was coined to contrast to the old term hardware . In contrast to hardware, software is intangible, meaning it "cannot be touched". Software is also sometimes used in a more that operates databases, providing storage, access, security, backup and other facilities. Database management systems can be categorized according to the database model A database model or database schema is the structure or format of a database, described in a formal language supported by the database management system, In other words, a "database model" is the application of a data model when used in conjunction with a database management system that they support, such as relational The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by E.F. Codd or XML An XML database is a data persistence software system that allows data to be stored in XML format. This data can then be queried, exported and serialized into the desired format, the type(s) of computer they support, such as a server cluster or a mobile phone, the query language Query languages are computer languages used to make queries into databases and information systems(s) that access the database, such as SQL SQL , often referred to as Structured Query Language, is a database computer language designed for managing data in relational database management systems (RDBMS), and originally based upon relational algebra. Its scope includes data insert, query, update and delete, schema creation and modification, and data access control. SQL was one of the or XQuery XQuery is a query and functional programming language that is designed to query collections of XML data, performance trade-offs, such as maximum scale or maximum speed or others. Some DBMS cover more than one entry in these categories, e.g., supporting multiple query languages.

Components of DBMS

Most DBMS as of 2009^[update] implement a relational model.^[2] Other DBMS systems, such as Object DBMS, offer specific features for more specialized requirements. Their components are similar, but not identical.

RDBMS components

• Sublanguages— Relational DBMS (RDBMS) include Data Definition Language A Data Definition Language is a computer language for defining data structures. The term was first introduced in relation to the Codasyl database model, where the schema of the database was written in a Data Definition Language describing the records, fields, and "sets" making up the user Data Model. Initially it referred to a subset of (DDL) for defining the structure of the database, Data Control Language (DCL) for defining security/access controls, and Data Manipulation Language Data Manipulation Language is a family of computer languages used by computer programs and/or database users to retrieve, insert, delete and update data in a database (DML) for querying and updating data.
• Interface drivers—These drivers are code libraries that provide methods to prepare statements, execute statements, fetch results, etc. Examples include ODBC In computing, Open Database Connectivity provides a standard software interface for accessing database management systems (DBMS). The designers of ODBC aimed to make it independent of programming languages, database systems, and operating systems. Thus, any application can use ODBC to query data from a database, regardless of the platform it is on, JDBC, MySQL MySQL is a relational database management system that runs as a server providing multi-user access to a number of databases. MySQL is officially pronounced /maɪˌɛskjuːˈɛl/ ("My S-Q-L"), but is often pronounced /maɪˈsiːkwəl/ ("My Sequel"). It is named for original developer Michael Widenius's daughter My/PHP PHP: Hypertext Preprocessor is a widely used, general-purpose scripting language that was originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document. As a general-purpose, FireBird Firebird is an open source relational database management system that runs on GNU/Linux, Windows, and a variety of Unix platforms. The database forked from Borland's open source edition of InterBase in 2000, but since Firebird 1.5 the code has been largely rewritten/Python Python is a general-purpose high-level programming language whose design philosophy emphasizes code readability. Python aims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive. Its use of indentation for block delimiters is unusual among popular programming languages.
• SQL engine—This component interprets and executes the DDL, DCL, and DML statements. It includes three major components (compiler, optimizer, and executor).
• Transaction engine—Ensures that multiple SQL statements either succeed or fail as a group, according to application dictates.
• Relational engine—Relational objects such as Table, Index, and Referential integrity constraints are implemented in this component.
• Storage engine A database engine is the underlying software component that a database management system (DBMS) uses to create, retrieve, update and delete (see CRUD (acronym)) data from a database. One may command the database engine via the DBMS's own user interface, and sometimes through a network port—This component stores and retrieves data from secondary storage, as well as managing transaction commit and rollback, backup and recovery, etc.

ODBMS components

Object DBMS (ODBMS) has transaction and storage components that are analogous to those in an RDBMS. Some ODBMS handle DDL, DCL and update tasks differently. Instead of using sublanguages, they provide APIs for these purposes. They typically include a sublanguage and accompanying engine for processing queries with interpretive statements analogous to but not the same as SQL. Example object query languages are OQL Object Query Language is a query language standard for object-oriented databases modelled after SQL. OQL was developed by the Object Data Management Group (ODMG). Because of its overall complexity no vendor has ever fully implemented the complete OQL. OQL has influenced the design of some of the newer query languages like JDOQL and EJB QL, but, LINQ Language Integrated Query is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages, JDOQL, JPAQL and others. The query engine returns collections of objects instead of relational rows.

Types

Operational database

These databases store detailed data about the operations of an organization. They are typically organized by subject matter, process relatively high volumes of updates using transactions. Essentially every major organization on earth uses such databases. Examples include customer databases that record contact, credit, and demographic information about a business' customers, personnel databases that hold information such as salary, benefits, skills data about employees, manufacturing databases Enterprise resource planning is an integrated computer-based system used to manage internal and external resources including tangible assets, financial resources, materials, and human resources. It is a software architecture whose purpose is to facilitate the flow of information between all business functions inside the boundaries of the that record details about product components, parts inventory, and financial databases that keep track of the organization's money, accounting and financial dealings.

Data warehouse

Data warehouses A data warehouse is a repository of an organization's electronically stored data, designed to facilitate reporting and analysis archive historical data from operational databases and often from external sources such as market research firms. Often operational data undergoes transformation on its way into the warehouse, getting summarized, anonymized, reclassified, etc. The warehouse becomes the central source of data for use by managers and other end-users who may not have access to operational data. For example, sales data might be aggregated to weekly totals and converted from internal product codes to use UPC codes so that it can be compared with ACNielsen ACNielsen is a global marketing research firm, with worldwide headquarters in New York City. Regional headquarters for North America are located in Schaumburg, IL. As of May 2010, it's the part of The Nielsen Company data.

Analytical database

Analysts may do their work directly against a data warehouse, or create a separate analytic database for Online Analytical Processing. For example, a company might extract sales records for analyzing the effectiveness of advertising and other sales promotions at an aggregate level.

Distributed database

These are databases of local work-groups and departments at regional offices, branch offices, manufacturing plants and other work sites. These databases can include segments of both common operational and common user databases, as well as data generated and used only at a user’s own site.

End-user database

These databases consist of data developed by individual end-users. Examples of these are collections of documents in spreadsheets, word processing and downloaded files, or even managing their personal baseball card collection.

External database

These databases contain data collect for use across multiple organizations, either freely or via subscription. The Internet Movie Database The Internet Movie Database is an online database of information related to movies, actors, television shows, production crew personnel, video games, and most recently, fictional characters featured in visual entertainment media. IMDb launched on October 17, 1990, and in 1998 was acquired by Amazon.com is one example.

Hypermedia databases

The Worldwide web The World Wide Web, abbreviated as WWW and commonly known as the Web, is a system of interlinked hypertext documents accessed via the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them by using hyperlinks. Using concepts from earlier hypertext systems, British can be thought of as a database, albeit one spread across millions of independent computing systems. Web browsers A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content. Hyperlinks present in resources enable users to easily navigate their browsers to "process" this data one page at a time, while web crawlers and other software provide the equivalent of database indexes to support search and other activities.

Models

Post-relational database models

Products offering a more general data model than the relational model are sometimes classified as post-relational.^[3] Alternate terms include "hybrid database", "Object-enhanced RDBMS" and others. The data model in such products incorporates relations In SQL, a database language for relational databases, a relation variable is called a table but is not constrained by E.F. Codd Edgar Frank "Ted" Codd was a British computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases. He made other valuable contributions to computer science, but the relational model, a very influential general theory of data management, remains his's Information Principle, which requires that

all information in the database must be cast explicitly in terms of values in relations and in no other way^[4]

Some of these extensions to the relational model integrate concepts from technologies that pre-date the relational model. For example, they allow representation of a directed graph It differs from an ordinary or undirected graph, in that the latter is defined in terms of unordered pairs of vertices, which are usually called edges with trees In computer science, a tree is a widely-used data structure that emulates a hierarchical tree structure with a set of linked nodes on the nodes In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.

Some post-relational products extend relational systems with non-relational features. Others arrived in much the same place by adding relational features to pre-relational systems. Paradoxically, this allows products that are historically pre-relational, such as PICK The Pick operating system is a demand-paged, multiuser, virtual memory, time-sharing operating system based around a unique "multivalued" database. It is used primarily for business data processing. Although it started on a variety of minicomputers, the system and various implementations eventually spread to a large variety of and MUMPS MUMPS , or alternatively M, is a programming language created in the late 1960s, originally for use in the healthcare industry. It was designed for the production of multi-user database-driven applications. It predates C and most other popular languages in current usage, and has very different syntax and terminology, to make a plausible claim to be post-relational.

Object database models

In recent years^[update], the object-oriented Object-oriented programming is a programming paradigm that uses "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction, encapsulation, modularity, polymorphism, and paradigm has been applied in areas such as engineering and spatial databases, telecommunications and in various scientific domains. The conglomeration of object oriented programming and database technology led to this new kind of database. These databases attempt to bring the database world and the application-programming world closer together, in particular by ensuring that the database uses the same type system In computer science, a type system may be defined as a tractable syntactic framework for classifying phrases according to the kinds of values they compute. A type system associates types with each computed value. By examining the flow of these values, a type system attempts to prove that no type errors can occur. The type system in question as the application program. This aims to avoid the overhead (sometimes referred to as the impedance mismatch The object-relational impedance mismatch is a set of conceptual and technical difficulties that are often encountered when a relational database management system is being used by a program written in an object-oriented programming language or style; particularly when objects or class definitions are mapped in a straightforward way to database) of converting information between its representation in the database (for example as rows in tables) and its representation in the application program (typically as objects). At the same time, object databases attempt to introduce key ideas of object programming, such as encapsulation In computer science, information hiding is the principle of segregation of design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed. The protection involves providing a stable interface which protects the remainder of the program and polymorphism In computer science, polymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface. The concept of parametric polymorphism applies to both data types and functions. A function that can evaluate to or be applied to values of different types is known as a polymorphic function. A, into the world of databases.

A variety of these ways have been tried^{[by whom?]} for storing objects in a database. Some products have approached the problem from the application-programming side, by making the objects manipulated by the program persistent Persistence in computer science refers to the characteristic of state that outlives the process that created it. Without this capability, state would only exist in RAM, and would be lost when this RAM loses power, such as a computer shutdown.[citation needed]. This also typically requires the addition of some kind of query language, since conventional programming languages do not provide language-level functionality for finding objects based on their information content. Others^[which?] have attacked the problem from the database end, by defining an object-oriented data model for the database, and defining a database programming language A programming language is an artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine, to express algorithms precisely, or as a mode of human communication that allows full programming capabilities as well as traditional query facilities.

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