SQL (sometimes expanded as Structured Query Language) is a computer language used to create, retrieve, update and delete data from relational database management systems. SQL has been standardized by both ANSI and ISO. SQL is commonly spoken either as the names of the letters ess-cue-el (IPA: [??s?kju??l]), or like the word sequel (IPA: [?si?kw?l]). The official pronunciation of SQL according to ANSI is ess-cue-el. However, each of the major database products (or projects) containing the letters SQL has its own convention: MySQL is officially and commonly pronounced "My Ess Cue El"; PostgreSQL is expediently pronounced postgres (being the name of the predecessor to PostgreSQL); and Microsoft SQL Server is commonly spoken as Microsoft-sequel-server. 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 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.[citation needed] 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. 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). 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 V2 beat IBM's release of the System/38 to the market by a few weeks. Standardization SQL was adopted as a standard by ANSI (American National Standards Institute) in 1986 and ISO (International Organization for Standardization) in 1987. However, since the dissolution of the NIST data management standards program in 1996 there has been no certification for compliance with the SQL standard so vendors must be relied on to self-certify. The SQL standard is not freely available. SQL:2003 and SQL:2006 may be purchased from ISO or ANSI. A late draft of SQL:2003 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. Scope SQL is designed for a specific purpose: to query data contained in a relational database. SQL is a set-based, declarative programming language, not an imperative language such as C or BASIC. Language extensions such as Oracle Corporation's 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. Extensions to and variations of the standards exist. 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. PL/SQL, IBM's SQL PL (SQL Procedural Language) and Sybase / Microsoft's Transact-SQL are of a proprietary nature because the procedural programming language they present are non-standardized. 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 database 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. SQL keywords Queries The most common operation in SQL databases is the query, denoted with the SELECT keyword. SQL SELECT queries are declarative: * SELECT retrieves data from tables in a database. While often grouped with Data Manipulation Language statements, SELECT is considered by many to be separate from SQL DML. SELECT queries allow the user to specify a description of the desired result set, but it is left to the devices of the database management system (DBMS) to plan, optimize, and perform the physical operations necessary to produce that result set. A SQL query includes a list of columns to be included in the final result immediately following the SELECT keyword. An asterisk ("*") can also be used as a "wildcard" indicator to specify that all available columns of a table (or multiple tables) are to be returned. SELECT is the most complex statement in SQL, with several optional keywords and clauses: o The FROM clause indicates the source tables from which the data is to be drawn. The FROM clause can include optional JOIN clauses to join related tables to one another. o The WHERE clause includes a comparison predicate, which is used to narrow the result set. The WHERE clause eliminates all rows from the result set for which the comparison predicate does not evaluate to True. o The GROUP BY clause is used to combine rows with related values into elements of a smaller set of rows. o The HAVING clause is used to identify which of the "combined rows" (combined rows are produced when the query has a GROUP BY clause 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 BYand can include aggregate functions. o The ORDER BY clause is used to identify which columns are used to sort the resulting data. Unless an ORDER BY clause is included, the order of rows returned by SELECT is never guaranteed. 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. Example 1: SELECT * FROM books WHERE price > 100.00 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. 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 which could also be written as SELECT title, count(*) AS Authors FROM books NATURAL JOIN book_authors GROUP BY title under the precondition that book_number is the only common column name of the two tables and that a column named title only exists in books. 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 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 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. INSERT Example: INSERT INTO my_table (field1, field2, field3) VALUES ('test', 'N', NULL); UPDATE Example: UPDATE my_table SET field1 = 'updated value' WHERE field2 = 'N'; DELETE Example: DELETE FROM my_table WHERE field2 = 'N'; Transaction controls Transactions, 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; 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) ); 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. Other * ANSI-standard SQL supports double dash, --, 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 Criticisms of SQL 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 (joining all possible combinations), 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. * It is also possible to misconstruct a WHERE on an update or delete, thereby affecting more rows in a table than desired. * SQL—and the relational model as it is—offer no standard way for handling tree-structures, i.e. rows recursively referring other rows of the same table. Oracle offers a "CONNECT BY" clause, Microsoft offers recursive joins via Common Table Expressions, other solutions are database functions which use recursion and return a row set, as possible in PostgreSQL with PL/PgSQL. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Active Server Pages (ASP) is Microsoft's server-side script engine for dynamically-generated web pages. It is marketed as an add-on to Internet Information Services (IIS). Programming ASP websites is made easier by various built-in objects. Each object corresponds to a group of frequently-used functionality useful for creating dynamic web pages. In ASP 2.0 there are six such built-in objects: Application, ASPError, Request, Response, Server, and Session. Session, for example, is a cookie-based session object that maintains variables from page to page. Most ASP pages are written in VBScript, but any other Active Scripting engine can be selected instead by using the @Language directive or thesyntax. JScript (Microsoft's implementation of ECMAScript) is the other language that is usually available. PerlScript (a derivative of Perl) and others are available as third-party installable Active Scripting engines. InstantASP and ChiliASP are technologies that run ASP without Windows Operating System. There are large open source communities on the WWW, such as ASPNuke, which produce ASP scripts, components and applications to be used for free under certain license terms. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Adobe Dreamweaver, or simply Dreamweaver, is a web development tool, originally created by Macromedia. Initial versions of the application served as simple WYSIWYG HTML editors but more recent versions have incorporated notable support for many other web technologies such as CSS, JavaScript, and various server-side scripting frameworks. The software is available for both the Mac and Windows platforms, but can also be run on Unix-like platforms through the use of emulation software such as Wine. Dreamweaver is currently owned by Adobe Systems who acquired Macromedia in 2005. Contents Features As a WYSIWYG editor, Dreamweaver can hide the details of pages' HTML code from the user, making it possible for non-coders to create web pages and sites. A professional criticism of this approach is that it produces HTML pages whose file size and amount of HTML code is much larger than they should be, which can cause web browsers to perform poorly. This can be particularly true because the application makes it very easy to create table-based layouts. In addition, some web site developers have criticized Dreamweaver in the past for producing code that often does not comply with W3C standards though this has improved considerably in recent versions. Dreamweaver 8.0 (the version prior to the recently released 9.0 within CS3) performed poorly on the Acid2 Test, developed by the Web Standards Project. However, Macromedia has increased the support for CSS and other ways to lay out a page without tables in later versions of the application, with the ability to convert tables to layers and vice versa. Adobe Dreamweaver CS3 Adobe Dreamweaver CS3 Dreamweaver allows users to preview websites in many browsers, provided that they are installed on their computer. It also has some site management tools, such as the ability to find and replace lines of text or code by whatever parameters specified across the entire site, and a templatization feature for creating multiple pages with similar structures. The behaviors panel also enables use of basic JavaScript without any coding knowledge. With the advent of version MX, Macromedia incorporated dynamic content creation tools into Dreamweaver. In the spirit of HTML WYSIWYG tools, it allows users to connect to databases (such as MySQL and Microsoft Access) to filter and display content using scripting technologies such as Active Server Pages (ASP), ASP.NET, ColdFusion, JavaServer Pages (JSP), PHP, and more without any previous programming experience. Dreamweaver 8.0 also included support for WYSIWYG XSLT editing. Alternative solutions for web database application development are Alpha Five and FileMaker. Dreamweaver can use "Extensions" - small programs, which any web developer can write (usually in HTML and JavaScript). Extensions provide added functionality to the software for whoever wants to download and install them. Dreamweaver is supported by a large community of extension developers who make extensions available (both commercial and free) for most web development tasks from simple rollover effects to full-featured shopping carts.
|
