HTML editor

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An HTML editor is a software application for creating web pages. Although the HTML markup of a web page can be written with any text editor, specialised HTML editors can offer convenience and added functionality. For example, many HTML editors work not only with HTML, but also with related technologies such as CSS, XML and JavaScript or ECMAScript. In some cases they also manage communication with remote web servers via FTP and WebDAV, and version management systems such as CVS or Subversion.

Contents [hide]
1 Types
1.1 Text editors
1.2 Object editors
1.3 WYSIWYG HTML editors
1.3.1 Criticism of WYSIWYG editors
1.3.2 WYSIWYM editors
2 Valid HTML code
3 Difficulties in achieving WYSIWYG
4 Comparison of HTML editors
4.1 General information
4.2 Operating system support
4.3 Editor features
4.4 Web technology support
4.5 Image format support
5 See also
6 References
7 External links



[edit] Types
There are various forms of HTML editors: text, object and WYSIWYG (What You See Is What You Get) editors.


[edit] Text editors
Text (source) editors intended for use with HTML usually provide syntax highlighting. Templates, toolbars and keyboard shortcuts may quickly insert common HTML elements and structures. Wizards, tooltip prompts and auto-completion may help with common tasks.

Text HTML editors commonly include either built-in functions or integration with external tools for such tasks as source and version control, link-checking, code checking and validation, code cleanup and formatting, spell-checking, uploading by FTP or WebDAV, and structuring as a project.

Text editors require user understanding of HTML and any other web technologies the designer wishes to use like CSS, JavaScript and server-side scripting languages.


[edit] Object editors
Some editors allow alternate editing of the source text of objects in more visually organized modes than simple color highlighting, but in modes not considered WYSIWYG. Some WYSIWYG editors include the option of using palette windows that enable editing the text-based parameters of selected objects. These pallets allow either editing parameters in fields for each individual parameter, or text windows to edit the full group of source text for the selected object. They may include widgets to present and select options when editing parameters. Adobe GoLive provides an outline editor to expand and collapse HTML objects and properties, edit parameters, and view graphics attached to the expanded objects.


[edit] WYSIWYG HTML editors

Amaya HTML editor
Nvu HTML editor
Quanta Plus HTML editorSo-called WYSIWYG HTML editors provide an editing interface which resembles how the page will be displayed in a web browser. Most WYSIWYG editors also have a mode to edit HTML directly as described above. Because using a WYSIWYG editor does not require any HTML knowledge, they are easier for an average computer user to get started with.

The WYSIWYG view is achieved by embedding a layout engine based upon that used in a web browser. The layout engine will have been considerably enhanced by the editor's developers to allow for typing, pasting, deleting and moving the content. The goal is that, at all times during editing, the rendered result should represent what will be seen later in a typical web browser.


[edit] Criticism of WYSIWYG editors
WYSIWYG editors facilitate the generation of web pages by people with little experience or knowledge of HTML. Experienced hand coders are prone to criticize the editing technology for the neglectful editing habits of the person editing, analogous to faulting automobile technology for reckless driving. Because WYSIWYG editors make it easy to build web pages, the editing technology is often faulted for the inexperience of the person editing, analogous to faulting digital cameras for amateur photography. Because WYSIWYG editors make complex visual layouts easier to create, the editing technology is often faulted for problems due to complexity of the layout.

WYSIWYG editors are sometimes criticized for the following reasons:

Depending on the version, WYSIWYG may not automatically generate the most efficient HTML and CSS code. However the code can be edited or generated by hand in WYSIWYG editors. Although third-party optimizers offer solutions to problems from automatically generated code, many of them simply remove extra spaces, rather than looking into the code to remove unneeded structures like compilers do. Unless the optimizer operates as a plug-in for the editor, it cannot take the web author's optimal preferences into account when content is created, resulting in mis-optimized code.
WYSIWYG editors make it easier to create layouts with HTML tables as an alternative to or combined with CSS. This is not a criticism of WYSIWYG editors as much as it is a criticism of HTML tables. Table based layouts are considered less efficient to download than CSS. Tables add complexity and obfuscates the documents' structures, resulting in code that is more difficult to maintain in text editing modes than CSS. It should be noted that WYSIWYG editors allow for table-free layouts and CSS as well as text editors allow for HTML tables.
Users may be disappointed that the same page is rendered differently in different browsers, on various screen sizes, and on varying monitor settings. This criticism is also misplaced. With the exception of Microsoft Frontpage, WYSIWYG editors mainly use W3C standard code. There are many factors outside of the page designer's control that can affect this—the CSS specification and modern browsers even allow users to override a page author's settings. The inconsistent browser display problem is due to inconsistent web browser technology, not WYSIWYG editing technology. see Difficulties in achieving WYSIWYG below.
Documents edited visually without regard to semantic structure can be incomprehensible to search engines, audio and text-only browsers. This is also a criticism of editing habits, not WYSIWYG editing technology. It should also be noted that search engines and browser technology may adapt to the habits of people as well as people will adapt to machines. Artificial intelligence may eventually enable machines to recognize and correct even the rarest problems in both editing and browsing.

[edit] WYSIWYM editors
What You See Is What You Mean (WYSIWYM) is an alternative paradigm to the WYSIWYG editors above. Instead of focusing on the format or presentation of the document, it preserves the intended meaning of each element. For example, page headers, sections, paragraphs, etc. are labeled as such in the editing program, and displayed appropriately in the browser.


[edit] Valid HTML code
HTML is a structured markup language. There are certain rules on how HTML must be written if it is to conform to W3C standards for the World Wide Web. Following these rules means that web sites are accessible on all types and makes of computer, to able-bodied and disabled people, and also on wireless devices like mobile phones and PDAs, with their limited bandwidths and screen sizes.

Unfortunately most HTML documents on the web are not valid according to W3C standards. According to one study only about 1 out of 141 is valid. Even those syntactically correct documents may be inefficient due to an unnecessary use of repetition, or based upon rules that have been deprecated for some years. Current W3C recommendations on the use of CSS with HTML were first formalised by W3C in 1996[1] and have been revised and refined since then. See CSS, XHTML, W3C's current CSS recommendation and W3C's current HTML recommendation.

These guidelines emphasise the separation of content (HTML or XHTML) from style (CSS). This has the benefit of delivering the style information once for a whole site, not repeated in each page, let alone in each HTML element. WYSIWYG editor designers have been struggling ever since with how best to present these concepts to their users without confusing them by exposing the underlying reality. Modern WYSIWYG editors all succeed in this to some extent, but none of them has succeeded entirely.

People who use text editors can generally fix such problems immediately, once they become aware of them. People find it frustrating when such errors come from WYSIWYG editors.

However a web page was created or edited, WYSIWYG or by hand, in order to be successful among the greatest possible number of readers and viewers, as well as to maintain the 'worldwide' value of the Web itself it can be argued that, first and foremost, it should consist of valid markup and code. Some would argue that it should not be delivered by a designer to his or her customer, and not be considered ready for the World Wide Web, until its HTML and CSS syntax has been successfully validated using either the free W3C validator services (W3C HTML Validator and W3C CSS Validator) or some other trustworthy alternatives.

Others would argue[2] that publishing useful information, as soon as possible, should be first and foremost.

Whatever software tools are used to design, create and maintain web pages, there is little doubt that the quality of the underlying HTML is dependent on the skill of the person who works on the page. Some knowledge of HTML, CSS and other scripting languages as well as a familiarity with the current W3C recommendations in these areas will help any designer produce better web pages, with a WYSIWYG HTML editor and without[3].


[edit] Difficulties in achieving WYSIWYG
A given HTML document will have an inconsistent appearance on various platforms and computers for several reasons:

Different browsers and applications will render the same markup differently.
The same page may display slightly differently in Internet Explorer and Firefox on a high-resolution screen, but it will look very different in the perfectly valid text-only Lynx browser. It needs to be rendered differently again on a PDA, an internet-enabled television and on a mobile phone. Usability in a speech or braille browser, or via a screen-reader working with a conventional browser, will place demands on entirely different aspects of the underlying HTML. Printing the page, via different browsers and different printers onto various paper sizes, around the world, places other demands. With the correct use of modern HTML and CSS there is no longer any need to provide 'Printable page' links and then have to maintain two versions of the whole site. Nor is there any excuse for pages not fitting the user's preferred paper size and orientation, or wasting ink printing solid background colours unnecessarily, or wasting paper reproducing navigation panels that will be entirely useless once printed out[4].
Browsers and computer graphics systems have a range of user settings.
Resolution, font size, colour, contrast etc can all be adjusted at the user's discretion, and many modern browsers allow even more user control over page appearance[5]. All an author can do is suggest an appearance.
Web browsers, like all computer software, have bugs
They may not conform to current standards. It is hopeless to try to design Web pages around all of the common browsers current bugs: each time a new version of each browser comes out, a significant proportion of the World Wide Web would need re-coding to suit the new bugs and the new fixes! It is generally considered much wiser to design to standards, staying away from 'bleeding edge' features until they settle down, and then wait for the browser developers to catch up to your pages, rather than the other way round[6]. In this regard, no one can argue that CSS/XHTML is still 'cutting edge' as there is now widespread support available in common browsers for all the major features[7], even if many WYSIWYG and other editors have not yet entirely caught up[8].
What you see may be what most visitors get, but it is not guaranteed to be what everyone gets.


[edit] Comparison of HTML editors
This article or section is incomplete and may require expansion and/or cleanup.
Please improve the article, or discuss the issue on the talk page.It has been suggested that this section be split into a new article entitled Comparison of HTML editors. (Discuss)
The following tables compare general and technical information for a number of (purportedly) WYSIWYG HTML editors. Please see the individual products' articles for further information, and Comparison of text editors for information on text editors, many of which have features to assist with writing HTML. This article is not all-inclusive or necessarily up-to-date.


[edit] General information
This article or section needs to be updated.
Parts of this article or section have been identified as no longer being up to date.
Please update the article to reflect recent events, and remove this template when finished.

Basic general information about the software: creator/company, license/price etc.

Editor Version Creator Cost (USD) Software license
Amaya 9.53 W3C, INRIA Free W3C
Aptana Milestone 7 Aptana Free EPL
Dreamweaver 8 Adobe Systems (formerly Macromedia) US $399 Closed source
FrontPage 2003 Microsoft US $199 Closed source
HomeSite 5.5 Adobe Systems (formerly Macromedia) USD $99 EUR €123 Adobe Acrobat License
Microsoft Expression Web Designer 2006 Microsoft US $299 Closed source
Mozilla Composer 1.7.13 Mozilla Foundation Free MPL/GPL/LGPL
Nvu 1.0 Daniel Glazman Free MPL/GPL/LGPL
OPENBEXI HTML Builder 1.5 arcazj Free GPL
SeaMonkey Composer 1.1 SeaMonkey Council Free MPL/GPL/LGPL


[edit] Operating system support
This section is a stub. You can help by expanding it.

Editor Windows Mac OS X Mac OS 9 Linux BSD Unix
Amaya Yes Yes No Yes source only source only
Aptana Yes Yes No Yes No No
Mozilla Composer Yes Yes Dropped Yes Yes Yes
Dreamweaver Yes Yes Dropped No No No
Microsoft FrontPage Yes No No[1] No No No
Nvu [2] Yes Yes No? Yes Yes ?


[edit] Editor features
Editor FTP Upload Server-side scripting Shared editing Spell checking Templates
Amaya No No No Yes No
Aptana Yes No Yes No Some (i.e. Snippets)
Mozilla Composer Yes? No No Yes No
Dreamweaver Yes Yes Yes Yes Yes
Microsoft FrontPage Yes Yes Yes Yes Yes
Nvu Yes No No Yes Yes


[edit] Web technology support
Editor CSS2 Frames Java JavaScript XSLT XHTML † MathML XForms RSS Atom XPath
Amaya Partial No No No No Yes Yes No No No ?
Aptana Yes Yes Yes Yes No Yes No No No No No
Mozilla Composer No Yes No Yes No No No No No No
Dreamweaver Yes Yes No Yes Yes Yes Partial ? Yes ? Yes
FrontPage Partial Yes Yes Yes Yes Yes Yes No No No No
Nvu Yes No No Yes No Yes No No No No No


[edit] Image format support
Editor JPEG GIF PNG MNG TIFF SVG PDF
Amaya Yes Yes Yes ? Yes Yes No
Aptana Yes Yes Yes Yes (coming soon) Yes Depends Depends
Mozilla Composer Yes Yes Yes No No No No
Dreamweaver Yes Yes Yes No No No ?
FrontPage[3] Yes Yes Yes Depends Yes? Depends Depends
Nvu Yes Yes Yes No No No No


[edit] See also
List of HTML editors
Web template system
Website builder
Validator

[edit] References
^ A version for Mac OS was released around 1998, but has since been discontinued.
^ Nvu can be built successfully on any platform with the Netscape Portable Runtime.
^ Actual amount of supported formats depend on installed converter. FrontPage uses Microsoft Office converters.

[edit] External links
Starting with HTML + CSS - W3C tutorial
Online-HTML-Editor.org
KTML - online HTML editor
PageBreeze Free Visual HTML Editor
Retrieved from "http://en.wikipedia.org/wiki/HTML_editor"

SQL

From Wikipedia, the free encyclopedia
(Redirected from Sql)
Jump to: navigation, search
SQL
Paradigm:
multi-paradigm: object-oriented, functional, procedural
Appeared in:
1974
Designed by:
Donald D. Chamberlin and Raymond F. Boyce
Developer:
IBM
Typing discipline:
static, strong
Major implementations:
Many
SQL (commonly expanded to Structured Query Language — see History for the term's derivation) is the most popular computer language used to create, modify, retrieve and manipulate data from relational database management systems. The language has evolved beyond its original purpose to support object-relational database management systems. It is an ANSI/ISO standard.
SQL is commonly spoken either as the names of the letters ess-cue-el (IPA: [ˈɛsˈkjuˈɛl]), or like the word sequel (IPA: [ˈsiːkwəl]). Concerning the names of major database products (or projects) containing the letters SQL, each has its own convention: MySQL is officially and commonly pronounced "My Ess Cue El"; PostgreSQL is expediently pronounced postgres (which had been the predecessor to PostgreSQL); and Microsoft SQL Server is commonly spoken as Microsoft-sequel-server.
Contents[hide]
1 History
1.1 Standardization
2 Scope
2.1 Reasons for lack of portability
3 SQL keywords
3.1 Data retrieval
3.2 Data manipulation
3.3 Transaction Controls
3.4 Data definition
3.5 Data control
3.6 Other
4 Criticisms of SQL
5 Alternatives to SQL
6 See also
6.1 Database systems using SQL
6.2 SQL variants
7 References
8 External links
//

[edit] History
An influential paper, "A Relational Model of Data for Large Shared Data Banks", by Dr. Edgar F. Codd, was published in June, 1970 in the Association for Computing Machinery (ACM) journal, Communications of the ACM, although drafts of it were circulated internally within IBM in 1969. Codd's model became widely accepted as the definitive model for relational database management systems (RDBMS or RDMS).
During the 1970s, a group at IBM's San Jose research center developed a database system "System R" based upon, but not strictly faithful to, Codd's model. Structured English Query Language ("SEQUEL") was designed to manipulate and retrieve data stored in System R. The acronym SEQUEL was later condensed to SQL because the word 'SEQUEL' was held as a trademark by the Hawker-Siddeley aircraft company of the UK. Although SQL was influenced by Codd's work, Donald D. Chamberlin and Raymond F. Boyce at IBM were the authors of the SEQUEL language design.[1] Their concepts were published to increase interest in SQL.
The first non-commercial, relational, non-SQL database, Ingres, was developed in 1974 at U.C. Berkeley.
In 1978, methodical testing commenced at customer test sites. Demonstrating both the usefulness and practicality of the system, this testing proved to be a success for IBM. As a result, IBM began to develop commercial products based on their System R prototype that implemented SQL, including the System/38 (announced in 1978 and commercially available in August 1979), SQL/DS (introduced in 1981), and DB2 (in 1983).[1]
At the same time Relational Software, Inc. (now Oracle Corporation) saw the potential of the concepts described by Chamberlin and Boyce and developed their own version of a RDBMS for the Navy, CIA and others. In the summer of 1979 Relational Software, Inc. introduced Oracle V2 (Version2) for VAX computers as the first commercially available implementation of SQL. Oracle is often incorrectly cited as beating IBM to market by two years, when in fact they only beat IBM's release of the System/38 by a few weeks. Considerable public interest then developed; soon many other vendors developed versions, and Oracle's future was ensured.

[edit] Standardization
SQL was adopted as a standard by ANSI (American National Standards Institute) in 1986 and ISO (International Organization for Standardization) in 1987. ANSI has declared that the official pronunciation for SQL is /ɛs kjuː ɛl/, although many English-speaking database professionals still pronounce it as sequel.[citation needed]
The SQL standard has gone through a number of revisions:
Year
Name
Alias
Comments
1986
SQL-86
SQL-87
First published by ANSI. Ratified by ISO in 1987.
1989
SQL-89
Minor revision.
1992
SQL-92
SQL2
Major revision (ISO 9075).
1999
SQL:1999
SQL3
Added regular expression matching, recursive queries, triggers, non-scalar types and some object-oriented features. (The last two are somewhat controversial and not yet widely supported.)
2003
SQL:2003

Introduced XML-related features, window functions, standardized sequences and columns with auto-generated values (including identity-columns).
The SQL standard is not freely available. SQL:2003 may be purchased from ISO or ANSI. A late draft is available as a zip archive from Whitemarsh Information Systems Corporation. The zip archive contains a number of PDF files that define the parts of the SQL:2003 specification.

[edit] Scope

The neutrality of this article or section may be compromised by weasel words.You can help Wikipedia by improving weasel-worded statements.
SQL is defined by both ANSI and ISO.
Extensions to and variations of the standards exist: Oracle Corporation's PL/SQL, IBM's SQL PL (SQL Procedural Language) and Sybase / Microsoft's Transact-SQL, which are of a proprietary nature. Commercial implementations commonly omit support for basic features of the standard, such as the DATE or TIME data types, preferring variations of their own. SQL code can rarely be ported between database systems without major modifications, in contrast to ANSI C or ANSI Fortran, which can usually be ported from platform to platform without major structural changes.
SQL is designed for a specific, limited purpose — querying data contained in a relational database. As such, it is a set-based, declarative computer language rather than an imperative language such as C or BASIC which, being general-purpose, are designed to solve a much broader set of problems.
Language extensions such as PL/SQL bridge this gap to some extent by adding procedural elements, such as flow-of-control constructs. Another approach is to allow programming language code to be embedded in and interact with the database. For example, Oracle and others include Java in the database, and SQL Server 2005 allows any .NET language to be hosted within the database server process, while PostgreSQL allows functions to be written in a wide variety of languages, including Perl, Tcl, and C.

[edit] Reasons for lack of portability
There are several reasons for this lack of portability between database systems:
The complexity and size of the SQL standard means that most databases do not implement the entire standard.
The standard does not specify database behavior in several important areas (e.g. indexes), leaving it up to implementations of the standard to decide how to behave.
The SQL standard precisely specifies the syntax that a conforming database system must implement. However, the standard's specification of the semantics of language constructs is less well-defined, leading to areas of ambiguity.
Many database vendors have large existing customer bases; where the SQL standard conflicts with the prior behavior of the vendor's database, the vendor may be unwilling to break backward compatibility.

[edit] SQL keywords
SQL keywords fall into several groups.

[edit] Data retrieval
The most frequently used operation in transactional databases is the data retrieval operation. When restricted to data retrieval commands, SQL acts as a declarative language.
SELECT is used to retrieve zero or more rows from one or more tables in a database. In most applications, SELECT is the most commonly used Data Manipulation Language command. In specifying a SELECT query, the user specifies a description of the desired result set, but they do not specify what physical operations must be executed to produce that result set. Translating the query into an efficient query plan is left to the database system, more specifically to the query optimizer.
Commonly available keywords related to SELECT include:
FROM is used to indicate from which tables the data is to be taken, as well as how the tables JOIN to each other.
WHERE is used to identify which rows to be retrieved, or applied to GROUP BY. WHERE is evaluated before the GROUP BY.
GROUP BY is used to combine rows with related values into elements of a smaller set of rows.
HAVING is used to identify which of the "combined rows" (combined rows are produced when the query has a GROUP BY keyword or when the SELECT part contains aggregates), are to be retrieved. HAVING acts much like a WHERE, but it operates on the results of the GROUP BY and hence can use aggregate functions.
ORDER BY is used to identify which columns are used to sort the resulting data.
Data retrieval is very often combined with data projection; usually it isn't the verbatim data stored in primitive data types that a user is looking for or a query is written to serve. Often the data needs to be expressed differently from how it's stored. SQL allows a wide variety of formulas included in the select list to project data. A common example would be:
SELECT UnitCost * Quantity As TotalCost FROM Orders Example 1:
SELECT * FROM books
WHERE price > 100.00 and price < 150.00
ORDER BY title
This is an example that could be used to get a list of expensive books. It retrieves the records from the books table that have a price field which is greater than 100.00 and less than 150.00. The result is sorted alphabetically by book title. The asterisk (*) means to show all columns of the books table. Alternatively, specific columns could be named. Example 2:
SELECT books.title, count(*) AS Authors
FROM books
JOIN book_authors
ON books.book_number = book_authors.book_number
GROUP BY books.title
Example 2 shows both the use of multiple tables in a join, and aggregation (grouping). This example shows how many authors there are per book. Example output may resemble: Title Authors
---------------------- -------
SQL Examples and Guide 3
The Joy of SQL 1
How to use Wikipedia 2
Pitfalls of SQL 1
How SQL Saved my Dog 1

[edit] Data manipulation
First there are the standard Data Manipulation Language (DML) elements. DML is the subset of the language used to add, update and delete data.
INSERT is used to add zero or more rows (formally tuples) to an existing table.
UPDATE is used to modify the values of a set of existing table rows.
MERGE is used to combine the data of multiple tables. It is something of a combination of the INSERT and UPDATE elements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called an "upsert".
DELETE removes zero or more existing rows from a table. Example:
INSERT INTO my_table (field1, field2, field3) VALUES ('test', 'N', NULL);
UPDATE my_table SET field1 = 'updated value' WHERE field2 = 'N';
DELETE FROM my_table WHERE field2 = 'N';

[edit] Transaction Controls
Transaction, if available, can be used to wrap around the DML operations.
BEGIN WORK (or START TRANSACTION, depending on SQL dialect) can be used to mark the start of a database transaction, which either completes completely or not at all.
COMMIT causes all data changes in a transaction to be made permanent.
ROLLBACK causes all data changes since the last COMMIT or ROLLBACK to be discarded, so that the state of the data is "rolled back" to the way it was prior to those changes being requested.
COMMIT and ROLLBACK interact with areas such as transaction control and locking. Strictly, both terminate any open transaction and release any locks held on data. In the absence of a BEGIN WORK or similar statement, the semantics of SQL are implementation-dependent.Example:
BEGIN WORK;
UPDATE inventory SET quantity = quantity - 3 WHERE item = 'pants';
COMMIT;

[edit] Data definition
The second group of keywords is the Data Definition Language (DDL). DDL allows the user to define new tables and associated elements. Most commercial SQL databases have proprietary extensions in their DDL, which allow control over nonstandard features of the database system. The most basic items of DDL are the CREATE,ALTER,RENAME,TRUNCATE and DROP commands.
CREATE causes an object (a table, for example) to be created within the database.
DROP causes an existing object within the database to be deleted, usually irretrievably.
TRUNCATE deletes all data from a table (non-standard, but common SQL command).
ALTER command permits the user to modify an existing object in various ways -- for example, adding a column to an existing table. Example:
CREATE TABLE my_table (
my_field1 INT,
my_field2 VARCHAR (50),
my_field3 DATE NOT NULL,
PRIMARY KEY (my_field1, my_field2)
);

[edit] Data control
The third group of SQL keywords is the Data Control Language (DCL). DCL handles the authorization aspects of data and permits the user to control who has access to see or manipulate data within the database. Its two main keywords are:
GRANT — authorizes one or more users to perform an operation or a set of operations on an object.
REVOKE — removes or restricts the capability of a user to perform an operation or a set of operations. Example:
GRANT SELECT, UPDATE ON my_table TO some_user, another_user.

[edit] Other
ANSI-standard SQL supports -- as a single line comment identifier (some extensions also support curly brackets or C-style /* comments */ for multi-line comments). Example:
SELECT * FROM inventory -- Retrieve everything from inventory table
Some SQL servers allow User Defined Functions

[edit] Criticisms of SQL

The neutrality of this article is disputed.Please see the discussion on the talk page.
Technically, SQL is a declarative computer language for use with "SQL databases". Theorists and some practitioners note that many of the original SQL features were inspired by, but in violation of, the relational model for database management and its tuple calculus realization. Recent extensions to SQL achieved relational completeness, but have worsened the violations, as documented in The Third Manifesto.
In addition, there are also some criticisms about the practical use of SQL:
Implementations are inconsistent and, usually, incompatible between vendors. In particular date and time syntax, string concatenation, nulls, and comparison case sensitivity often vary from vendor-to-vendor.
The language makes it too easy to do a Cartesian join, which results in "run-away" result sets when WHERE clauses are mistyped. Cartesian joins are so rarely used in practice that requiring an explicit CARTESIAN keyword may be warranted.

[edit] Alternatives to SQL
A distinction should be made between alternatives to relational query languages and alternatives to SQL. The list below are proposed alternatives to SQL, but are still (nominally) relational. See navigational database for alternatives to relational.
IBM Business System 12 (IBM BS12)
Tutorial D
TQL - Luca Cardelli (May not be relational)
Top's Query Language - A draft language influenced by IBM BS12. Tentatively renamed to SMEQL to avoid confusion with similar projects called TQL.
Hibernate Query Language[2] (HQL) - A Java-based tool that uses modified SQL
Quel introduced in 1974 by the U.C. Berkeley Ingres project.
Object Query Language - Object Data Management Group.
Datalog
LINQ

[edit] See also

[edit] Database systems using SQL
Comparison of relational database management systems
Comparison of truly relational database management systems
Comparison of object-relational database management systems
List of relational database management systems
List of object-relational database management systems
List of hierarchical database management systems

[edit] SQL variants
Comparison of SQL syntax

[edit] References
^ Donald D. Chamberlin and Raymond F. Boyce, 1974. "SEQUEL: A structured English query language", International Conference on Management of Data, Proceedings of the 1974 ACM SIGFIDET (now SIGMOD) workshop on Data description, access and control, Ann Arbor, Michigan, pp. 249–264
Discussion on alleged SQL flaws (C2 wiki)
Web page about FSQL: References and links.
Galindo J., Urrutia A., Piattini M., "Fuzzy Databases: Modeling, Design and Implementation". Idea Group Publishing Hershey, USA, 2005.

[edit] External links
SQL Basics
The 1995 SQL Reunion: People, Projects, and Politics (early history of SQL)
SQL:2003, SQL/XML and the Future of SQL (webcast and podcast with Jim Melton, editor of the SQL standard)
A Gentle Introduction to SQL at SQLzoo
SQL Help and Tutorial
The SQL Language (PostgreSQL specific details included)

Wikibooks Programming has more about this subject:
SQL
SQL Exercises. SQL DML Help and Tutorial
SQL Tutorial.
A free SQL cookbook for all SQL dialects
Online Interactive SQL Tutorials
How well Oracle, DB2, MSSQL support the SQL Standard
SQL Tutorial
The sbVB DataBase course - A free course on software development using cross-platform C++ and SQL (for any Relational Database, such as Oracle, MSSQL, PostgreSQL, MySQL, DB2, Informix and others)
Topics in database management systems (DBMS) ( view • talk • edit )
ConceptsDatabase Database model Relational database Relational model Relational algebra Primary key - Foreign key - Surrogate key - Superkey - Candidate keyDatabase normalization Referential integrity Relational DBMS Distributed DBMS ACID
ObjectsTrigger View Table Cursor Log Transaction Index Stored procedure Partition
Topics in SQLSelect Insert Update Merge Delete Join Union Create Drop
Implementations of database management systems
Types of implementationsRelational Flat file Deductive Dimensional Hierarchical Object oriented Temporal XML data storesProductsApache Derby Berkeley DB Caché db4o dBASE Firebird Helix DB2 Informix Ingres InterBase Linter Microsoft SQL Server Microsoft Access MySQL OpenLink Virtuoso Oracle PostgreSQL SQLite Sybase IQ Sybase Teradata TimesTen Visual FoxPro Comparison - relational Comparison - object-relational
ComponentsQuery language Query optimizer Query plan ODBC JDBCListsList of object-oriented database management systemsList of relational database management systems

Object-oriented programming

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Object-oriented programming (OOP) is a programming paradigm that uses "objects" to design applications and computer programs. It utilizes several techniques from previously established paradigms, including inheritance, modularity, polymorphism, and encapsulation. Even though it originated in the 1960s, OOP was not commonly used in mainstream software application development until the 1990s. Today, many popular programming languages (such as Java, JavaScript, C#, C++, Python, PHP, Ruby and Objective-C) support OOP.
Object-oriented programming's roots reach all the way back to the creation of the Simula programming language in the 1960s, when the nascent field of software engineering had begun to discuss the idea of a software crisis. As hardware and software became increasingly complex, how could software quality be maintained? Object-oriented programming in part addresses this problem by strongly emphasizing modularity in software.[1]
Object-oriented programming may be seen as a collection of cooperating objects, as opposed to a traditional view in which a program may be seen as a collection of functions, or simply as a list of instructions to the computer. In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects. Each object can be viewed as an independent little machine with a distinct role or responsibility.[2]
Object-oriented programming is intended to promote greater flexibility and maintainability in programming, and is widely popular in large-scale software engineering. By virtue of its strong emphasis on modularity, object oriented code is intended to be simpler to develop and easier to understand later on, lending itself to more direct analysis, coding, and understanding of complex situations and procedures than less modular programming methods.
Contents[hide]
1 Fundamental concepts
1.1 Prototype-based programming
2 History
2.1 OOP in scripting
3 Problems and patterns
3.1 Gang of Four design patterns
3.2 Object-orientation and databases
3.3 Matching real world
4 Formal definition
5 Criticism
6 See also
7 Further reading
8 Notes
9 References
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[edit] Fundamental concepts
A survey of nearly 40 years of computing literature by Deborah J. Armstrong[3] identified a number of "quarks," or fundamental concepts, identified in the strong majority of definitions of OOP. They are:
Class
A class defines the abstract characteristics of a thing, including the thing's characteristics (its attributes or properties) and the things it can do (its behaviors or methods or features). For example, the class Dog would consist of traits shared by all dogs, for example breed, fur color, and the ability to bark. Classes provide modularity and structure in an object-oriented computer program. A class should typically be recognizable to a non-programmer familiar with the problem domain, meaning that the characteristics of the class should make sense in context. Also, the code for a class should be relatively self-contained. Collectively, the properties and methods defined by a class are called members.
Object
A particular instance of a class. The class of Dog defines all possible dogs by listing the characteristics that they can have; the object Lassie is one particular dog, with particular versions of the characteristics. A Dog has fur; Lassie has brown-and-white fur. In programmer jargon, the object Lassie is an instance of the Dog class. The set of values of the attributes of a particular object is called its state.
Method
An object's abilities. Lassie, being a Dog, has the ability to bark. So bark() is one of Lassie's methods. She may have other methods as well, for example sit() or eat(). Within the program, using a method should only affect one particular object; all Dogs can bark, but you need one particular dog to do the barking.
Message passing
"The process by which an object sends data to another object or asks the other object to invoke a method."[3]
Inheritance
In some cases, a class will have "subclasses," more specialized versions of a class. For example, the class Dog might have sub-classes called Collie, Chihuahua, and GoldenRetriever. In this case, Lassie would be an instance of the Collie subclass. Subclasses inherit attributes and behaviors from their parent classes, and can introduce their own. Suppose the Dog class defines a method called bark() and a property called furColor. Each of its sub-classes (Collie, Chihuahua, and GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once. Each subclass can alter its inherited traits. So, for example, the Collie class might specify that the default furColor for a collie is brown-and-white. The Chihuahua subclass might specify that the bark() method is high-pitched by default. Subclasses can also add new members. The Chihuahua subclass could add a method called tremble(). So an individual chihuahua instance would use a high-pitched bark() from the Chihuahua subclass, which in turn inherited the usual bark() from Dog. The chihuahua object would also have the tremble() method, but Lassie would not, because she is a Collie, not a Chihuahua. In fact, inheritance is an "is-a" relationship: Lassie is a Collie. A Collie is a Dog. Thus, Lassie inherits the members of both Collies and Dogs. When an object or class inherits its traits from more than one ancestor class, and neither of these ancestors is an ancestor of the other, then it's called multiple inheritance. For example, independent classes could define Dogs and Cats, and a Chimera object could be created from these two which inherits all the (multiple) behaviour of cats and dogs. This is not always supported, as it can be hard both to implement and to use well.
Encapsulation
Conceals the exact details of how a particular class works from objects that use its code or send messages to it. So, for example, the Dog class has a bark() method. The code for the bark() method defines exactly how a bark happens (e.g., by inhale() and then exhale(), at a particular pitch and volume). Timmy, Lassie's friend, however, does not need to know exactly how she barks. Encapsulation is achieved by specifying which classes may use the members of an object. The result is that each object exposes to any class a certain interface — those members accessible to that class. The reason for encapsulation is to prevent clients of an interface from depending on those parts of the implementation that are likely to change in future, thereby allowing those changes to be made more easily, that is, without changes to clients. For example, an interface can ensure that puppies can only be added to an object of the class Dog by code in that class. Members are often specified as public, protected and private, determining whether they are available to all classes, sub-classes or only the defining class. Some languages go further: Java uses the protected keyword to restrict access also to classes in the same package, C# and VB.NET reserve some members to classes in the same assembly using keywords internal (C#) or Friend (VB.NET), and Eiffel allows one to specify which classes may access any member.
Abstraction
Simplifying complex reality by modeling classes appropriate to the problem, and working at the most appropriate level of inheritance for a given aspect of the problem. For example, Lassie the Dog may be treated as a Dog much of the time, a Collie when necessary to access Collie-specific attributes or behaviors, and as an Animal (perhaps the parent class of Dog) when counting Timmy's pets.
Polymorphism
Polymorphism is behavior that varies depending on the class in which the behavior is invoked, that is, two or more classes can react differently to the same message. For example, if a Dog is commanded to speak() this may elicit a Bark; if a Pig is commanded to speak() this may elicit an Oink.

[edit] Prototype-based programming
Not all of the above concepts are to be found in all object-based programming languages. In particular, prototype-based programming does not typically use classes. As a result, a significantly different yet analogous terminology is used to define the concepts of object and instance in these languages.

[edit] History
The concept of objects and instances in computing had its first major breakthrough with the PDP-1 system at MIT which was probably the earliest example of capability based architecture. Another early example was Sketchpad made by Ivan Sutherland in 1963; however, this was an application and not a programming paradigm.
Objects as programming entities were introduced in the 1960's in Simula 67, a programming language designed for making simulations, created by Ole-Johan Dahl and Kristen Nygaard of the Norwegian Computing Center in Oslo. (Reportedly, the story is that they were working on ship simulations, and were confounded by the combinatorial explosion of how the different attributes from different ships could affect one another. The idea occurred to group the different types of ships into different classes of objects, each class of objects being responsible for defining its own data and behavior.) Such an approach was a simple extrapolation of concepts earlier used in analog programming. On analog computers, such direct mapping from real-world phenomena/objects to analog phenomena/objects (and conversely), was (and is) called 'simulation.' Simula not only introduced the notion of classes, but also of instances of classes, which is probably the first explicit use of those notions.
The Smalltalk language, which was developed at Xerox PARC in the 1970's, introduced the term Object-oriented programming to represent the pervasive use of objects and messages as the basis for computation. Smalltalk creators were influenced by the ideas introduced in Simula 67, but Smalltalk was designed to be a fully dynamic system in which classes could be created and modified dynamically rather than statically as in Simula 67. The ideas in Simula 67 were also used in many other languages, from derivatives of Lisp to Pascal.
Object-oriented programming developed as the dominant programming methodology during the mid-1980s, largely due to the influence of C++, an extension of the C programming language. Its dominance was further cemented by the rising popularity of graphical user interfaces, for which object-oriented programming is well-suited. An example of a closely related dynamic GUI library and OOP language can be found in the Cocoa frameworks on Mac OS X, written in Objective C, an object-oriented, dynamic messaging extension to C based on Smalltalk. OOP toolkits also enhanced the popularity of "event-driven programming" (although this concept is not limited to OOP). Some feel that association with GUIs (real or perceived) was what propelled OOP into the programming mainstream.
OOP also became increasingly popular for developing computer games during the 1990s. As the complexity of games grew, as faster hardware became more widely available and compilers (especially C++) matured, more and more games and their engines were written in OOP languages. Prominent C++ examples [4] include the Doom III engine, Starcraft, Diablo, Warcraft III and World of Warcraft. Since almost all video games feature virtual environments which contain many, often thousands of objects that interact with each other in complex ways, OOP languages are particularly suited for game development.
At ETH Zürich, Niklaus Wirth and his colleagues had also been investigating such topics as data abstraction and modular programming. Modula-2 included both, and their succeeding design, Oberon included a distinctive approach to object orientation, classes, and such. The approach is unlike Smalltalk, and very unlike C++.
Object-oriented features have been added to many existing languages during that time, including Ada, BASIC, Lisp, Fortran, Pascal, and others. Adding these features to languages that were not initially designed for them often led to problems with compatibility and maintainability of code. "Pure" object-oriented languages, on the other hand, lacked features that many programmers had come to depend upon. To bridge this gap, many attempts have been made to create new languages based on object-oriented methods but allowing some procedural features in "safe" ways. Bertrand Meyer's Eiffel was an early and moderately successful language with those goals.
In the past decade Java has emerged in wide use partially because of its similarity to C and to C++, but perhaps more importantly because of its implementation using a virtual machine that is intended to run code unchanged on many different platforms. This last feature has made it very attractive to larger development shops with heterogeneous environments. Microsoft's .NET initiative has a similar objective and includes/supports several new languages, or variants of older ones.
More recently, a number of languages have emerged that are primarily object-oriented yet compatible with procedural methodology, such as Python and Ruby. Besides Java, probably the most commercially important recent object-oriented languages are Visual Basic .NET and C# designed for Microsoft's .NET platform.
Just as procedural programming led to refinements of techniques such as structured programming, modern object-oriented software design methods include refinements such as the use of design patterns, design by contract, and modeling languages (such as UML).

[edit] OOP in scripting
In recent years, object-oriented programming has become especially popular in scripting programming languages. Python and Ruby are scripting languages built on OOP principles, while Perl and PHP have been adding object oriented features since Perl 5 and PHP 4.
The Document Object Model of HTML, XHTML, and XML documents on the Internet have bindings to the popular JavaScript/ECMAScript language. JavaScript is perhaps the best known prototype-based programming language.

[edit] Problems and patterns
There are a number of programming challenges which a developer encounters regularly in object-oriented design. There are also widely accepted solutions to these problems. The best known are the design patterns codified by Gamma et al, but in a more general sense the term "design patterns" can be used to refer to any general, repeatable solution to a commonly occurring problem in software design. Some of these commonly occurring problems have implications and solutions particular to object-oriented development.

[edit] Gang of Four design patterns
Main article: Design Patterns
Design Patterns: Elements of Reusable Object-Oriented Software is an influential book published in 1995 by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, sometimes casually called the "Gang of Four." Along with exploring the capabilities and pitfalls of object-oriented programming, it describes 23 common programming problems and patterns for solving them.
As of April 2005, the book was in its 32nd printing. While it can make for dense reading, even for experienced programmers, and has been superseded in practice by a spate of more recent, more accessible books, it is regarded as an important source not only for design patterns but also for the object-oriented design guidelines in its initial chapter.

[edit] Object-orientation and databases
Main articles: Object-Relational impedance mismatch, Object-relational mapping, and Object database
Both object-oriented programming and relational database management systems (RDBMSs) are extremely common in software today. Since relational databases don't store objects directly (though some RDBMSs have object-oriented features to approximate this), there is a general need to bridge the two worlds. There are a number of widely used solutions to this problem. One of the most common is object-relational mapping, as found in libraries like ADO.NET, Java Data Objects, and Ruby on Rails.
There are also object databases which can be used to replace RDBMSs, but these have not been as commercially successful as RDBMSs.

[edit] Matching real world
OOP can be used to translate from real-world phenomena to program elements (and vice versa). OOP was even invented for the purpose of physical modelling in the Simula-67 programming language. However, not everyone agrees that direct real-world mapping is facilitated by OOP, or is even a worthy goal; Bertrand Meyer argues in Object-Oriented Software Construction[5] that a program is not a model of the world but a model of a model of some part of the world; "Reality is a cousin twice removed"

[edit] Formal definition
There have been several attempts at formalizing the concepts used in object-oriented programming. The following concepts and constructs have been used as interpretations of OOP concepts:
coalgebraic datatypes
existential quantification and modules
recursion
records and record extensions
F-bounded polymorphism
Attempts to find a consensus definition or theory behind objects have not proven very successful, and often diverge widely. For example, some definitions focus on mental activities, and some on mere program structuring. One of the simpler definitions is that OOP is the act of using "map" data structures or arrays that can contain functions and pointers to other maps, all with some syntactic and scoping sugar on top. Inheritance can be performed by cloning the maps (sometimes called "prototyping").

[edit] Criticism
Much of the criticism of OOP comes from proponents of relational databases or relational-like techniques such as set theory, functional programming[6], and logical programming.
One study by Potok et. al. [1] has showed no significant difference in productivity between OOP and procedural approaches.

[edit] See also
Object-oriented programming language
Aspect-oriented programming
Procedural programming
Object oriented analysis and design
Software componentry
Interface description language
List of object-oriented programming terms
Refactoring
CORBA
DCOM
Object-relational mapping
Object-Relational impedance mismatch
Object database
Object-oriented image classification

[edit] Further reading

Wikibooks has more about this subject:
Computer programming/Object oriented programming

At Wikiversity you can learn about Object-oriented programming at:
Topic:Object-Oriented Programming
Abadi, Martin; Luca Cardelli. A Theory of Objects. Springer-Verlag. ISBN 0-387-94775-2.
Abelson, Harold; Gerald Jay Sussman,. Structure and Interpretation of Computer Programs. The MIT Press. ISBN 0-262-01153-0.
Booch, Grady. Object-Oriented Analysis and Design with Applications. Addison-Wesley. ISBN 0-8053-5340-2.
Eeles, Peter; Oliver Sims. Building Business Objects. John Wiley & Sons. ISBN 0-471-19176-0.
Gamma, Erich; Richard Helm, Ralph Johnson, John Vlissides. Design Patterns: Elements of Reusable Object Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
Harmon, Paul; William Morrissey. The Object Technology Casebook - Lessons from Award-Winning Business Applications. John Wiley & Sons. ISBN 0-471-14717-6.
Jacobson, Ivar. Object-Oriented Software Engineering: A Use Case-Driven Approach. Addison-Wesley. ISBN 0-201-54435-0.
Kay, Alan. The Early History of Smalltalk.
Rumbaugh, James; Michael Blaha, William Premerlani, Frederick Eddy, William Lorensen. Object-Oriented Modeling and Design. Prentice Hall. ISBN 0-13-629841-9.
Taylor, David A.. Object-Oriented Information Systems - Planning and Implementation. John Wiley & Sons. ISBN 0-471-54364-0.
"OOP Better in Theory than in Practice" by Richard Mansfield

[edit] Notes
^ Meyer, chapter 3
^ Booch, chapter 2
^ a b Armstrong, "The Quarks of Object-Oriented Development." In descending order of popularity, the "quarks" are: Inheritance, Object, Class, Encapsulation, Method, Message Passing, Polymorphism, Abstraction
^ C++ Applications, by Bjarne Stroustrup, the author of C++
^ Meyer, Second Edition, p. 230
^ Paul GRAHAM.

[edit] References
Armstrong, Deborah J. (February 2006). "The Quarks of Object-Oriented Development". Communications of the ACM 49 (2): 123–128. ISSN 0001-0782. Retrieved on 2006-08-08.
Meyer, Bertrand (1997). Object-Oriented Software Construction. Prentice Hall. ISBN 0-13-629155-4.