SQL 29004 225995040 2008-07-16T10:57:42Z 76.25.245.32 /* Language elements */ {{otheruses4|Structured Query Language|the airport with IATA code SQL|San Carlos Airport}} {{Infobox programming language | name = SQL | logo = | paradigm = [[multi-paradigm programming language|Multi-paradigm]] | year = 1974 | designer = [[Donald D. Chamberlin]] and [[Raymond F. Boyce]] | developer = [[IBM]] | latest_release_version = SQL:2006 | latest_release_date = 2006 | latest_test_version = | latest_test_date = | turing-complete = No | typing = [[static typing|static]], [[strong typing|strong]] | implementations = [[List of relational database management systems|Many]] | dialects = SQL-86, SQL-89, [[SQL-92]], [[SQL:1999]], [[SQL:2003]], [[SQL:2006]] | influenced_by = [[Datalog]] | influenced = [[CQL]], [[Language Integrated Query|LINQ]], [[Windows PowerShell]] | operating_system = [[Cross-platform]] | license = | website = }} <!-- Please do not change the "sequal" pronounciation back to "incorrect" without discussing and building consensus in the talk pages first. If you change it without consensus, it will be reverted. Also do not delete or change this note without similarly building consensus. --> '''SQL''' ('''Structured Query Language''') ({{pronEng|ɛskjuːˈɛl }} officially, although the unofficial pronunciation {{IPA|/ˈsiːkwəl/}} is often used ([[#Standardization|see below]])), is a database [[computer language]] designed for the retrieval and management of [[data]] in [[relational database management system]]s (RDBMS), database schema creation and modification, and database object access control management.<ref name="chapple">{{cite web | last = Chapple | first = Mike | title = SQL Fundamentals | work = About.com: Databases | publisher = About.com | url = http://databases.about.com/od/sql/a/sqlfundamentals.htm?terms=SQL | format = HTML | accessdate = 2007-06-10 }}</ref><ref name="IBM-sql">{{cite web | title = Structured Query Language (SQL) | publisher = International Business Machines | url = http://publib.boulder.ibm.com/infocenter/db2luw/v9/index.jsp?topic=/com.ibm.db2.udb.admin.doc/doc/c0004100.htm | format = HTML | date = October 27, 2006 | accessdate = 2007-06-10 }}</ref> SQL is a standard interactive and programming language for querying and modifying data and managing databases. Although SQL is both an [[American National Standards Institute|ANSI]] and an [[International Organization for Standardization|ISO]] standard, many database products support SQL with proprietary extensions to the standard language. The core of SQL is formed by a command language that allows the retrieval, insertion, updating, and deletion of data, and performing management and administrative functions. SQL also includes a [[Call Level Interface]] (SQL/CLI) for accessing and managing data and databases remotely. The first version of SQL was developed at [[IBM]] by [[Donald D. Chamberlin]] and [[Raymond F. Boyce]] in the early 1970s. This version, initially called '''SEQUEL''', was designed to manipulate and retrieve data stored in IBM's original relational database product, [[System R]]. The SQL language was later formally [[Standardization|standard]]ized by the [[American National Standards Institute]] (ANSI) in 1986. Subsequent versions of the SQL standard have been released as [[International Organization for Standardization]] (ISO) standards. Originally designed as a [[declarative programming|declarative]] query and data manipulation language, variations of SQL have been created by SQL [[database management system|database management system (DBMS)]] vendors that add procedural constructs, [[Control_flow|control-of-flow]] statements, user-defined data types, and various other language extensions. With the release of the SQL:1999 standard, many such extensions were formally adopted as part of the SQL language via the SQL Persistent Stored Modules (SQL/PSM) portion of the standard. Common criticisms of SQL include a perceived lack of cross-platform portability between vendors, inappropriate handling of missing data (''see [[Null (SQL)]]''), and unnecessarily complex and occasionally ambiguous language grammar and semantics. == History == During the 1970s, a group at [[IBM Almaden Research Center|IBM San Jose Research Laboratory]] developed the [[System R]] relational database management system, based on the model introduced by [[Edgar F. Codd]] in his influential [[academic publishing|paper]], '''A Relational Model of Data for Large Shared Data Banks'''.<ref name="codd-relational-model">{{cite journal | last = Codd | first = E.F. | title = A Relational Model of Data for Large Shared Data Banks | journal = Communications of the ACM | volume = 13 | issue = No. 6 | pages = pp. 377–387 | publisher = Association for Computing Machinery | date = June 1970 | url = http://www.acm.org/classics/nov95/toc.html | accessdate = 2007-06-09 | doi = 10.1145/362384.362685}}</ref> Donald D. Chamberlin and Raymond F. Boyce of IBM subsequently created the '''Structured English Query Language''' (SEQUEL) to manipulate and manage data stored in System R.<ref name="chamberlin-boyce-sequel">{{cite journal | last = Chamberlin | first = Donald D. | coauthors = Boyce, Raymond F. | title = SEQUEL: A Structured English Query Language | journal = Proceedings of the 1974 ACM SIGFIDET Workshop on Data Description, Access and Control | pages = pp. 249–264 | publisher = Association for Computing Machinery | date = 1974 | url = http://www.almaden.ibm.com/cs/people/chamberlin/sequel-1974.pdf | accessdate = 2007-06-09 }}</ref> The acronym SEQUEL was later changed to SQL because "SEQUEL" was a [[trademark]] of the [[United Kingdom|UK-based]] [[Hawker Siddeley]] aircraft company.<ref name="oppel-databases">{{cite book | last = Oppel | first = Andy | title = Databases Demystified | publisher = McGraw-Hill Osborne Media | date = March 1, 2004 | location = [[San Francisco, CA]] | pages = pp. 90-91 | url = http://www.mhprofessional.com/product.php?cat=112&isbn=0072253649 | id = ISBN 0-07-225364-9 }}</ref> The first non-commercial non-SQL RDBMS, [[Ingres]], was developed in 1974 at the [[University of California, Berkeley|U.C. Berkeley]]. Ingres implemented a query language known as [[QUEL query languages|QUEL]], which was later supplanted in the marketplace by SQL.<ref name="oppel-databases"/> In the late 1970s, Relational Software, Inc. (now [[Oracle Corporation]]) saw the potential of the concepts described by Codd, Chamberlin, and Boyce and developed their own SQL-based [[RDBMS]] with aspirations of selling it to the [[United States Navy|U.S. Navy]], [[Central Intelligence Agency|CIA]], and other [[Federal government of the United States|government]] agencies. In the summer of [[1979]], Relational Software, Inc. introduced the first commercially available implementation of SQL, [[Oracle database|Oracle]] V2 (Version2) for [[VAX]] computers. ''Oracle V2'' beat IBM's release of the [[System/38]] RDBMS to market by a few weeks.{{Fact|date=June 2007}} After testing SQL at customer test sites to determine the usefulness and practicality of the system, IBM began developing commercial products based on their System R prototype including [[System/38]], [[SQL/DS]], and [[IBM DB2|DB2]], which were commercially available in [[1979]], [[1981]], and [[1983]], respectively.<ref name="IBM-history">{{cite web | title = History of IBM, 1978 | work = IBM Archives | publisher = IBM | url = http://www-03.ibm.com/ibm/history/history/year_1978.html | format = HTML | accessdate = 2007-06-09 }}</ref> === Standardization === SQL was adopted as a standard by [http://special.lib.umn.edu/findaid/xml/cbi00168.xml ANSI] in 1986 and ISO in 1987. In the original SQL standard, ANSI declared that the official pronunciation for SQL is "es queue el".<ref>{{cite web | last = Chapple | first = Mike | title = SQL Fundamentals | work = About.com | publisher = About.com, A New York Times Company | date = ? | url = http://databases.about.com/od/sql/a/sqlfundamentals.htm | format = HTML | accessdate = 2007-08-30 }}</ref> However, many English-speaking database professionals still use the nonstandard<ref>{{cite book |title=Understanding the New SQL: A Complete Guide |last=Melton |first=Jim |coauthors=Alan R Simon |year=1993 |publisher=Morgan Kaufmann |isbn=1558602453 |pages=536 |quote=chapter 1.2 What is SQL? SQL (correctly pronounced "ess cue ell," instead of the somewhat common "sequel"), is a...}}</ref> pronunciation {{IPA|/ˈsiːkwəl/}} (like the word "sequel"). Until [[1996]], the [[National Institute of Standards and Technology|National Institute of Standards and Technology (NIST)]] data management standards program was tasked with certifying SQL DBMS compliance with the SQL standard. In 1996, however, the NIST data management standards program was dissolved, and vendors are now relied upon to self-certify their products for compliance.<ref>{{cite web | last = Doll | first = Shelley | title = Is SQL a Standard Anymore? | work = TechRepublic's Builder.com | publisher = TechRepublic | date = June 19, 2002 | url = http://builder.com.com/5100-6388-1046268.html | format = HTML | accessdate = 2007-06-09 }}</ref> The SQL standard has gone through a number of revisions, as shown below: {| class=wikitable ! Year !! Name !! width=125 | Alias !! Comments |- valign=top | 1986 || SQL-86 || SQL-87 || First published by ANSI. Ratified by ISO in 1987. |- valign=top | [[1989]] || SQL-89 || [[Federal Information Processing Standard|FIPS]] 127-1 || Minor revision, adopted as FIPS 127-1. |- valign=top | [[1992]] || [[SQL-92]] || SQL2, FIPS 127-2 || Major revision (ISO 9075), ''Entry Level'' SQL-92 adopted as FIPS 127-2. |- valign=top | [[1999]] || [[SQL:1999]] || SQL3 || Added regular expression matching, recursive queries, [[Database trigger|triggers]], support for procedural and control-of-flow statements, non-scalar types, and some object-oriented features. |- valign=top | [[2003]] || [[SQL:2003]] ||&nbsp; || Introduced [[XML]]-related features, ''window functions'', standardized sequences, and columns with auto-generated values (including identity-columns). |- valign=top | [[2006]] || [[SQL:2006]] || &nbsp; || ISO/IEC 9075-14:2006 defines ways in which SQL can be used in conjunction with XML. It defines ways of importing and storing XML data in an SQL database, manipulating it within the database and publishing both XML and conventional SQL-data in XML form. In addition, it provides facilities that permit applications to integrate into their SQL code the use of [[XQuery]], the XML Query Language published by the World Wide Web Consortium ([[W3C]]), to concurrently access ordinary SQL-data and XML documents. |} The SQL standard is not freely available. SQL:2003 and SQL:2006 may be purchased from ISO or ANSI. A [http://www.wiscorp.com/sql_2003_standard.zip late draft] of SQL:2003 is freely available as a [[ZIP file format|zip]] archive, however, from [http://www.wiscorp.com/SQLStandards.html Whitemarsh Information Systems Corporation]. The zip archive contains a number of [[PDF]] files that define the parts of the SQL:2003 specification. == Scope and extensions == === Procedural extensions === SQL is designed for a specific purpose: to query data contained in a [[relational database]]. SQL is a [[set]]-based, [[Declarative programming|declarative]] query language, not an [[Imperative programming|imperative language]] such as [[C (programming language)|C]] or [[BASIC programming language|BASIC]]. However, there are extensions to Standard SQL which add [[procedural programming language]] functionality, such as control-of-flow constructs. These are: {| class="wikitable" |- ! Source ! Common<br>Name ! Full Name |- |align="center"| ANSI/ISO Standard |align="center"| [[SQL/PSM]] | SQL/Persistent Stored Modules |- |align="center"| IBM |align="center"| [[SQL PL]] | SQL Procedural Language (implements SQL/PSM) |- |align="center"| [[Microsoft]]/<br>[[Sybase]] |align="center"| [[T-SQL]] | Transact-SQL |- |align="center"| [[MySQL]] |align="center"| SQL/PSM | SQL/Persistent Stored Module (as in ISO SQL:2003) |- |align="center"| [[Oracle Corporation|Oracle]] |align="center"| [[PL/SQL]] | Procedural Language/SQL (based on [[Ada]]) |- |align="center"| [[PostgreSQL]] |align="center"| [[PL/pgSQL]] | Procedural Language/PostgreSQL Structured Query Language (based on Oracle PL/SQL) |- |align="center"| [[PostgreSQL]] |align="center"| [[PL/PSM]] | Procedural Language/Persistent Stored Modules (implements SQL/PSM) |} In addition to the standard SQL/PSM extensions and proprietary SQL extensions, procedural and [[object-oriented programming language|object-oriented]] programmability is available on many SQL platforms via DBMS integration with other languages. The SQL standard defines [[SQL/JRT]] extensions (SQL Routines and Types for the Java Programming Language) to support [[Java (programming language)|Java]] code in SQL databases. [[SQL Server 2005]] uses the [[SQLCLR]] (SQL Server Common Language Runtime) to host managed [[Microsoft .NET|.NET]] assemblies in the database, while prior versions of SQL Server were restricted to using unmanaged extended stored procedures which were primarily written in C. Other database platforms, like MySQL and Postgres, allow functions to be written in a wide variety of languages including [[Perl]], [[Python (programming language)|Python]], [[Tcl]], and C. === Additional extensions === SQL:2003 also defines several additional extensions to the standard to increase SQL functionality overall. These extensions include: The [[SQL/CLI]], or '''Call-Level Interface''', extension is defined in ISO/IEC 9075-3:2003. This extension defines common interfacing components (structures and procedures) that can be used to execute SQL statements from applications written in other programming languages. The SQL/CLI extension is defined in such a way that SQL statements and SQL/CLI procedure calls are treated as separate from the calling application's source code. The [[SQL/MED]], or '''Management of External Data''', extension is defined by ISO/IEC 9075-9:2003. SQL/MED provides extensions to SQL that define foreign-data wrappers and datalink types to allow SQL to manage external data. External data is data that is accessible to, but not managed by, an SQL-based DBMS. The [[SQL/OLB]], or '''Object Language Bindings''', extension is defined by ISO/IEC 9075-10:2003. SQL/OLB defines the syntax and symantics of [[SQLJ]], which is SQL embedded in Java. The standard also describes mechanisms to ensure binary portability of SQLJ applications, and specifies various Java packages and their contained classes. The [[SQL/Schemata]], or '''Information and Definition Schemas''', extension is defined by ISO/IEC 9075-11:2003. SQL/Schemata defines the Information Schema and Definition Schema, providing a common set of tools to make SQL databases and objects self-describing. These tools include the SQL object identifier, structure and integrity constraints, security and authorization specifications, features and packages of ISO/IEC 9075, support of features provided by SQL-based DBMS implementations, SQL-based DBMS implementation information and sizing items, and the values supported by the DBMS implementations.<ref name="ISO/IEC 9075-11:2003">{{Citation | title = ISO/IEC 9075-11:2003: Information and Definition Schemas (SQL/Schemata) | pages = p. 1 | year = 2003 }}</ref> The SQL/JRT, or '''SQL Routines and Types for the Java Programming Language''', extension is defined by ISO/IEC 9075-13:2003. SQL/JRT specifies the ability to invoke static Java methods as routines from within SQL applications. It also calls for the ability to use Java classes as SQL structured user-defined types. The [[SQL/XML]], or '''XML-Related Specifications''', extension is defined by ISO/IEC 9075-14:2003. SQL/XML specifies SQL-based extensions for using XML in conjunction with SQL. The XML data type is introduced, as well as several routines, functions, and XML-to-SQL data type mappings to support manipulation and storage of XML in an SQL database. The SQL/PSM, or '''Persistent Stored Modules''', extension is defined by ISO/IEC 9075-4:2003. SQL/PSM standardizes procedural extensions for SQL, including flow of control, condition handling, statement condition signals and resignals, cursors and local variables, and assignment of expressions to variables and parameters. In addition, SQL/PSM formalizes declaration and maintenance of persistent database language routines (e.g., "stored procedures"). == Language elements == [[Image:SQL_ANATOMY_wiki.svg|thumb|275px|This chart shows several of the SQL language elements that compose a single statement.]] The SQL language is sub-divided into several language elements, including: * ''Statements'' which may have a persistent effect on schemas and data, or which may control transactions, program flow, connections, sessions, or diagnostics. * ''Queries'' which retrieve data based on specific criteria. * ''Expressions'' which can produce either [[scalar (computing)|scalar]] values or [[table (database)|tables]] consisting of [[column (database)|columns]] and [[row (database)|rows]] of data. * ''Predicates'' which specify conditions that can be evaluated to SQL [[ternary logic|three-valued logic (3VL)]] Boolean truth values and which are used to limit the effects of statements and queries, or to change program flow. * ''Clauses'', which are in some cases optional, constituent components of statements and queries.<ref>ANSI/ISO/IEC International Standard (IS). Database Language SQL—Part 2: Foundation (SQL/Foundation). 1999</ref> * [[Whitespace (computer science)|Whitespace]] is generally ignored in SQL statements and queries, making it easier to format SQL code for readability. * SQL statements also include the [[semicolon]] (";") statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar. === Queries === The most common operation in SQL databases is the query, which is performed with the declarative <code>[[Select (SQL)|SELECT]]</code> keyword. <code>SELECT</code> retrieves data from a specified [[Table (database)|table]], or multiple related tables, in a database. While often grouped with [[Data Manipulation Language|Data Manipulation Language (DML)]] statements, the standard <code>SELECT</code> query is considered separate from SQL DML, as it has no persistent effects on the data stored in a database. Note that there are some platform-specific variations of <code>SELECT</code> that can persist their effects in a database, such as the <code>SELECT INTO</code> syntax that exists in some databases.<ref name="ms-sql-select-into">{{cite web | title = INTO Clause (Transact-SQL) | work = SQL Server 2005 Books Online | publisher = Microsoft | date = 2007 | url = http://msdn2.microsoft.com/en-us/library/ms188029(SQL.90).aspx | format = HTML | accessdate = 2007-06-17}}</ref> SQL 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|database management system (DBMS)]] to [[query plan|plan]], [[query optimizer|optimize]], and perform the physical operations necessary to produce that result set in as efficient a manner as possible. An SQL query includes a list of columns to be included in the final result immediately following the <code>SELECT</code> keyword. An asterisk ("<code>*</code>") can also be used as a "wildcard" indicator to specify that all available columns of a table (or multiple tables) are to be returned. <code>SELECT</code> is the most complex statement in SQL, with several optional keywords and clauses, including: * The <code>[[From (SQL)|FROM]]</code> clause which indicates the source table or tables from which the data is to be retrieved. The <code>FROM</code> clause can include optional <code>[[Join (SQL)|JOIN]]</code> clauses to join related tables to one another based on user-specified criteria. * The <code>[[Where (SQL)|WHERE]]</code> clause includes a comparison predicate, which is used to restrict the number of rows returned by the query. The <code>WHERE</code> clause is applied before the <code>GROUP BY</code> clause. The <code>WHERE</code> clause eliminates all rows from the result set where the comparison predicate does not evaluate to True. * The <code>[[Group by (SQL)|GROUP BY]]</code> clause is used to combine, or group, rows with related values into elements of a smaller set of rows. <code>GROUP BY</code> is often used in conjunction with SQL aggregate functions or to eliminate duplicate rows from a result set. * The <code>[[Having (SQL)|HAVING]]</code> clause includes a comparison predicate used to eliminate rows after the <code>GROUP BY</code> clause is applied to the result set. Because it acts on the results of the <code>GROUP BY</code> clause, aggregate functions can be used in the <code>HAVING</code> clause predicate. * The <code>[[Order by (SQL)|ORDER BY]]</code> clause is used to identify which columns are used to sort the resulting data, and in which order they should be sorted (options are ascending or descending). The order of rows returned by an SQL query is never guaranteed unless an <code>ORDER BY</code> clause is specified. The following is an example of a <code>SELECT</code> query that returns a list of expensive books. The query retrieves all rows from the ''Book'' table in which the ''price'' column contains a value greater than 100.00. The result is sorted in ascending order by ''title''. The asterisk (*) in the ''select list'' indicates that all columns of the ''Book'' table should be included in the result set. <source lang="sql"> SELECT * FROM Book WHERE price > 100.00 ORDER BY title; </source> The example below demonstrates the use of multiple tables in a join, grouping, and aggregation in an SQL query, by returning a list of books and the number of authors associated with each book. <source lang="sql"> SELECT Book.title, count(*) AS Authors FROM Book JOIN Book_author ON Book.isbn = Book_author.isbn GROUP BY Book.title; </source> Example output might resemble the following: Title Authors ---------------------- ------- SQL Examples and Guide 3 The Joy of SQL 1 How to use Wikipedia 2 Pitfalls of SQL 1 Under the precondition that ''[[isbn]]'' is the only common column name of the two tables and that a column named ''title'' only exists in the ''Books'' table, the above query could be rewritten in the following form: <source lang="sql"> SELECT title, count(*) AS Authors FROM Book NATURAL JOIN Book_author GROUP BY title; </source> However, many vendors either do not support this approach, or it requires certain column naming conventions. Thus, it is less common in practice. Data retrieval is very often combined with [[Projection (relational algebra)|data projection]] when the user is looking for calculated values and not just the verbatim data stored in [[primitive type|primitive data types]], or when the data needs to be expressed in a form that is different from how it's stored. SQL allows the use of expressions in the ''select list'' to project data, as in the following example which returns a list of books that cost more than 100.00 with an additional ''sales_tax'' column containing a sales tax figure calculated at 6% of the ''price''. <source lang="sql"> SELECT isbn, title, price, price * 0.06 AS sales_tax FROM Book WHERE price > 100.00 ORDER BY title; </source> Some modern day SQL queries may include extra WHERE statements that are conditional to each other. They may look like this example: <source lang="sql"> SELECT isbn, title, price, date FROM Book WHERE price > 100.00 AND (date = '16042004' or date = '16042005') ORDER BY title; </source> [[Universal quantification]] is not explicitly supported by sql, and must be worked out as a negated [[existential quantification]].<ref>M. Negri, G. Pelagatti, L. Sbattella (1989) ''[http://portal.acm.org/citation.cfm?id=63224.68822&coll=GUIDE&dl=GUIDE Semantics and problems of universal quantification in SQL]''</ref><ref>Claudio Fratarcangeli (1991) ''[http://portal.acm.org/citation.cfm?id=126482.126484&coll=GUIDE&dl=GUIDE&CFID=5934371&CFTOKEN=55309005 Technique for universal quantification in SQL]''</ref><ref>Jalal Kawash ''[http://www.thefreelibrary.com/Complex+quantification+in+Structured+Query+Language+(SQL):+a+tutorial...-a0119901477 Complex quantification in Structured Query Language (SQL): a tutorial using relational calculus]'' - Journal of Computers in Mathematics and Science Teaching ISSN 0731-9258 Volume 23, Issue 2, 2004 AACE Norfolk, VA </ref> === 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: * <code>[[Insert (SQL)|INSERT]]</code> is used to add rows (formally [[tuple]]s) to an existing table, for example: <source lang="sql"> INSERT INTO My_table (field1, field2, field3) VALUES ('test', 'N', NULL); </source> * <code>[[Update (SQL)|UPDATE]]</code> is used to modify the values of a set of existing table rows, eg: <source lang="sql"> UPDATE My_table SET field1 = 'updated value' WHERE field2 = 'N'; </source> * <code>[[Delete (SQL)|DELETE]]</code> removes zero or more existing rows from a table, eg: <source lang="sql"> DELETE FROM My_table WHERE field2 = 'N'; </source> * <code>[[Merge (SQL)|MERGE]]</code> is used to combine the data of multiple tables. It is something of a combination of the <code>INSERT</code> and <code>UPDATE</code> elements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called an "[[upsert]]". === Transaction controls === Transactions, if available, can be used to wrap around the DML operations: * <code>START TRANSACTION</code> (or <code>[[Begin work (SQL)|BEGIN WORK]]</code>, or <code>BEGIN TRANSACTION</code>, depending on SQL dialect) can be used to mark the start of a [[database transaction]], which either completes entirely or not at all. * <code>[[Commit (SQL)|COMMIT]]</code> causes all data changes in a transaction to be made permanent. * <code>[[Rollback (SQL)|ROLLBACK]]</code> causes all data changes since the last <code>COMMIT</code> or <code>ROLLBACK</code> to be discarded, so that the state of the data is "rolled back" to the way it was prior to those changes being requested. Once the <code>COMMIT</code> statement has been executed, the changes ''cannot'' be rolled back. In other words, its meaningless to have <code>ROLLBACK</code> executed after <code>COMMIT</code> statement and vice versa. <code>COMMIT</code> and <code>ROLLBACK</code> 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 <code>START TRANSACTION</code> or similar statement, the semantics of SQL are implementation-dependent. Example: <i>A classic bank transfer of funds transaction.</i> <source lang="sql"> START TRANSACTION; UPDATE Account SET amount=amount-200 WHERE account_number=1234; UPDATE Account SET amount=amount+200 WHERE account_number=2345; IF ERRORS=0 COMMIT; IF ERRORS<>0 ROLLBACK; </source> === 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 <code>CREATE</code>, <code>ALTER</code>, <code>RENAME</code>, <code>TRUNCATE</code> and <code>DROP</code> statements: * <code>[[Create (SQL)|CREATE]]</code> causes an object (a table, for example) to be created within the database. * <code>[[Drop (SQL)|DROP]]</code> causes an existing object within the database to be deleted, usually irretrievably. *<code>[[Truncate (SQL)|TRUNCATE]]</code> deletes all data from a table (non-standard, but common SQL statement). *<code>[[Alter (SQL)|ALTER]]</code> statement permits the user to modify an existing object in various ways -- for example, adding a column to an existing table. Example: <source lang="sql"> CREATE TABLE My_table ( my_field1 INT, my_field2 VARCHAR (50), my_field3 DATE NOT NULL, PRIMARY KEY (my_field1, my_field2) ); </source> === 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: * <code>GRANT</code> authorizes one or more users to perform an operation or a set of operations on an object. * <code>REVOKE</code> removes or restricts the capability of a user to perform an operation or a set of operations. Example: <source lang="sql"> GRANT SELECT, UPDATE ON My_table TO some_user, another_user; </source> === Other === * ANSI-standard SQL supports double dash, <code>--</code>, as a single line [[comment (computer programming)|comment]] identifier (some extensions also support curly brackets or C style <code>/* comments */</code> for multi-line comments). Example: <source lang="sql"> SELECT * FROM Inventory -- Retrieve everything from inventory table </source> * Some SQL servers allow [[user-defined function]]s == 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 relational calculus|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 product|Cartesian]] join (joining all possible combinations), which results in "run-away" result sets when <code>WHERE</code> clauses are mistyped. Cartesian joins are so rarely used in practice that requiring an explicit <code>CARTESIAN</code> keyword may be warranted.<br> ''SQL 1992'' introduced the <code>CROSS JOIN</code> keyword that allows the user to make clear that a cartesian join is intended, but the shorthand "comma-join" with no predicate is still acceptable syntax. * It is also possible to misconstruct a <code>WHERE</code> on an update or delete, thereby affecting more rows in a table than desired. * The grammar of SQL is perhaps unnecessarily complex, borrowing a [[COBOL]]-like keyword approach, when a function-influenced syntax could result in more re-use of fewer grammar and syntax rules. This is perhaps due to IBM's early goal of making the language more English-like so that it is more approachable to those without a mathematical or programming background. (Predecessors to SQL were more mathematical.) === Reasons for lack of portability === Popular implementations of SQL commonly omit support for basic features of Standard SQL, such as the <code>DATE</code> or <code>TIME</code> data types, preferring variations of their own. As a result, SQL code can rarely be ported between database systems without modifications. 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. [[index (database)|indexes]], file storage...), 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]]. == 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) * [[D (data language specification)#Tutorial D|Tutorial D]] * [[Hibernate Query Language|Hibernate Query Language]] (HQL) - A Java-based tool that uses modified SQL * [[QUEL query languages|Quel]] introduced in 1974 by the U.C. Berkeley Ingres project. * [[Object Query Language]] * [[Datalog]] * [[.QL]] - object-oriented Datalog * [[Language Integrated Query|LINQ]] * [[QLC]] - Query Interface to Mnesia, ETS, Dets, etc ([[Erlang (programming language)|Erlang]] programming language) * [[4th Dimension (Software)|4D Query Language]] (4D QL) * [[Query_by_Example|QBE (Query By Example)]] created by Moshe Zloof, IBM 1977 * [[Aldat Relational Algebra]] and [[Domain algebra]] == See also == * [[Alphabetical list of programming languages]] * [[D (data language specification)]] * [[D4 (programming language)]] (an implementation of D) * [[Comparison of relational database management systems]] * [[Comparison of object-relational database management systems]] * [[List of relational database management systems]] * [[Hierarchical model]] * [[MUMPS]] == References == {{wikibooks|SQL}} {{wikibooks|SQL dialects reference}} {{reflist}} <div class="references-small"> *[http://www.acm.org/classics/nov95/toc.html "A Relational Model of Data for Large Shared Data Banks"] E. F. Codd, Communications of the ACM, Vol. 13, No. 6, June 1970, pp. 377-387. *[[c2:SqlFlaws|Discussion on alleged SQL flaws]] (C2 wiki) </div> == External Links == * [http://www.cbi.umn.edu/oh/display.phtml?id=317 Oral history interview with Donald D. Chamberlin] [[Charles Babbage Institute]] In this oral history Chamberlin recounts his early life, his education at Harvey Mudd College and Stanford University, and his work on relational database technology. Chamberlin was a member of the System R research team and, with [[Raymond F. Boyce]], developed the [[SQL]] database language. Chamberlin also briefly discusses his more recent research on XML query languages. * [http://special.lib.umn.edu/findaid/xml/cbi00168.xml American National Standards Institute. X3H2 Records, 1978-1995] [[Charles Babbage Institute]] Collection documents the H2 committee’s development of the NDL and SQL standards. {{Databases}} [[Category:Declarative programming languages]] [[Category:Query languages]] [[Category:SQL| ]] [[Category:Databases]] [[Category:IBM software]] [[Category:Data modeling languages]] [[Category:Articles with example SQL code]] [[ar:لغة الاستعلامات البنيوية]] [[bn:এসকিউএল]] [[be:SQL]] [[bs:SQL]] [[br:Areg SQL]] [[bg:SQL]] [[ca:Structured Query Language]] [[cs:SQL]] [[da:Structured Query Language]] [[de:SQL]] [[es:SQL]] [[eo:SQL]] [[eu:SQL]] [[fa:اس‌کیوال]] [[fr:Structured Query Language]] [[ga:Teanga Struchtúrtha Iarratas]] [[ko:SQL]] [[hr:SQL]] [[id:SQL]] [[ia:SQL]] [[is:SQL]] [[it:SQL]] [[he:SQL]] [[ku:SQL]] [[lv:SQL]] [[lt:SQL]] [[hu:SQL]] [[ms:SQL]] [[cdo:SQL]] [[nl:SQL]] [[ja:SQL]] [[no:Structured Query Language]] [[pl:SQL]] [[pt:SQL]] [[ro:SQL]] [[ru:SQL]] [[sk:SQL]] [[sl:SQL]] [[sr:SQL]] [[fi:SQL]] [[sv:SQL]] [[th:ภาษาสอบถามเชิงโครงสร้าง]] [[vi:SQL]] [[tr:SQL]] [[uk:SQL]] [[zh:SQL]]