SQL

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SQL
Paradigm(s)Multi-paradigm
Designed byDonald D. Chamberlin
Raymond F. Boyce
DeveloperISO/IEC
Appeared in1974
Stable releaseSQL:2011 / 2011
Typing disciplineStatic, strong
Major implementationsMany
DialectsSQL-86, SQL-89, SQL-92, SQL:1999, SQL:2003, SQL:2006, SQL:2008, SQL:2011
Influenced byDatalog
InfluencedCQL, LINQ, SOQL, Windows PowerShell,[1] JPQL, jOOQ
OSCross-platform
File format(s)File format details
Filename extension.sql
Internet media typeapplication/sql[2][3]
Developed byISO/IEC
Initial release1986 (1986)
Latest releaseSQL:2898  / 2011; 3 years ago (2011)
Type of formatDatabase
StandardISO/IEC 9075
Open format?Yes
 
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This article is about the database language. For the IATA code, see San Carlos Airport (California).
SQL
Paradigm(s)Multi-paradigm
Designed byDonald D. Chamberlin
Raymond F. Boyce
DeveloperISO/IEC
Appeared in1974
Stable releaseSQL:2011 / 2011
Typing disciplineStatic, strong
Major implementationsMany
DialectsSQL-86, SQL-89, SQL-92, SQL:1999, SQL:2003, SQL:2006, SQL:2008, SQL:2011
Influenced byDatalog
InfluencedCQL, LINQ, SOQL, Windows PowerShell,[1] JPQL, jOOQ
OSCross-platform
File format(s)File format details
Filename extension.sql
Internet media typeapplication/sql[2][3]
Developed byISO/IEC
Initial release1986 (1986)
Latest releaseSQL:2898  / 2011; 3 years ago (2011)
Type of formatDatabase
StandardISO/IEC 9075
Open format?Yes

SQL (/ˈɛs kjuː ˈɛl/,[4] or /ˈskwəl/; Structured Query Language[5][6][7][8]) is a special-purpose programming language designed for managing data held in a relational database management system (RDBMS).

Originally based upon relational algebra and tuple relational calculus, SQL consists of a data definition language and a data manipulation language. The scope of SQL includes data insert, query, update and delete, schema creation and modification, and data access control. Although SQL is often described as, and to a great extent is, a declarative language (4GL), it also includes procedural elements.

SQL was one of the first commercial languages for Edgar F. Codd's relational model, as described in his influential 1970 paper, "A Relational Model of Data for Large Shared Data Banks."[9] Despite not entirely adhering to the relational model as described by Codd, it became the most widely used database language.[10][11]

SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987.[12] Since then, the standard has been enhanced several times with added features. Despite these standards, code is not completely portable among different database systems, which can lead to vendor lock-in. The different makers do not perfectly adhere to the standard, for instance by adding extensions, and the standard itself is sometimes ambiguous.

History[edit]

SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce in the early 1970s.[13] This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasi-relational database management system, System R, which a group at IBM San Jose Research Laboratory had developed during the 1970s.[13] The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company.[14]

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 U.S. Navy, Central Intelligence Agency, and other U.S. government agencies. In June 1979, Relational Software, Inc. introduced the first commercially available implementation of SQL, Oracle V2 (Version2) for VAX computers.

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 DB2, which were commercially available in 1979, 1981, and 1983, respectively.[15]

Syntax[edit]

Language elements[edit]

A chart showing several of the SQL language elements that compose a single statement

The SQL language is subdivided into several language elements, including:

Operators[edit]

OperatorDescriptionExample
=Equal toAuthor = 'Alcott'
<>Not equal to (most DBMS also accept != instead of <>)Dept <> 'Sales'
>Greater thanHire_Date > '2012-01-31'
<Less thanBonus < 50000.00
>=Greater than or equalDependants >= 2
<=Less than or equalRate <= 0.05
BETWEENBetween an inclusive rangeCost BETWEEN 100.00 AND 500.00
LIKEMatch a character patternFirst_Name LIKE 'Will%'
INEqual to one of multiple possible valuesDeptCode IN (101, 103, 209)
IS or IS NOTCompare to null (missing data)Address IS NOT NULL
IS NOT DISTINCT FROMIs equal to value or both are nulls (missing data)Debt IS NOT DISTINCT FROM - Receivables
ASUsed to change a field name when viewing resultsSELECT employee AS 'department1'

Conditional (CASE) expressions[edit]

SQL has the case/when/then/else/end expression, which was introduced in SQL-92. In its most general form, which is called a "searched case" in the SQL standard, it works like else if in other programming languages:

 CASE WHEN n > 0            THEN 'positive'       WHEN n < 0            THEN 'negative'      ELSE 'zero' END 

SQL tests WHEN conditions in the order they appear in the source. If the source does not specify an ELSE expression, SQL defaults to ELSE NULL. An abbreviated syntax—called "simple case" in the SQL standard—mirrors switch statements:

 CASE n WHEN 1              THEN 'one'         WHEN 2             THEN 'two'         ELSE 'i cannot count that high' END 

This syntax uses implicit equality comparisons, with the usual caveats for comparing with NULL.

For the Oracle-SQL dialect, the latter can be shortened to an equivalent DECODE construct:

 SELECT DECODE(n, 1, 'one',  	         2, 'two',                  'i cannot count that high') FROM   some_table; 

The last value is the default; if none is specified, it also defaults to NULL. However, unlike the standard's "simple case", Oracle's DECODE considers two NULLs equal with each other.[17]

Queries[edit]

The most common operation in SQL is the query, which is performed with the declarative SELECT statement. SELECT retrieves data from one or more tables, or expressions. Standard SELECT statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax that exists in some databases.[18]

Queries allow the user to describe desired data, leaving the database management system (DBMS) responsible for planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.

A query includes a list of columns to include in the final result, immediately following the SELECT keyword. An asterisk ("*") can also be used to specify that the query should return all columns of the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:

The following is an example of a SELECT 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.

 SELECT *  FROM  Book  WHERE price > 100.00  ORDER BY title; 

The example below demonstrates a query of multiple tables, grouping, and aggregation, by returning a list of books and the number of authors associated with each book.

 SELECT Book.title AS Title,        COUNT(*) AS Authors  FROM  Book  JOIN  Book_author    ON  Book.isbn = Book_author.isbn  GROUP BY Book.title; 

Example output might resemble the following:

 Title                  Authors ---------------------- ------- SQL Examples and Guide 4 The Joy of SQL         1 An Introduction to SQL 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:

 SELECT title,        COUNT(*) AS Authors  FROM  Book   NATURAL JOIN Book_author  GROUP BY title; 

However, many vendors either do not support this approach, or require certain column naming conventions for natural joins to work effectively.

SQL includes operators and functions for calculating values on stored values. 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.

 SELECT isbn,        title,        price,        price * 0.06 AS sales_tax  FROM  Book  WHERE price > 100.00  ORDER BY title; 

Subqueries[edit]

Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases, the use of subqueries introduces a hierarchy in execution that can be useful or necessary. In the following example, the aggregation function AVG receives as input the result of a subquery:

 SELECT isbn,        title,        price  FROM  Book  WHERE price < (SELECT AVG(price) FROM Book)  ORDER BY title; 

A subquery can use values from the outer query, in which case it is known as a correlated subquery.

Since 1999 the SQL standard allows named subqueries called common table expression (named and designed after the IBM DB2 version 2 implementation; Oracle calls these subquery factoring). CTEs can also be recursive by referring to themselves; the resulting mechanism allows tree or graph traversals (when represented as relations), and more generally fixpoint computations.

Null and three-valued logic (3VL)[edit]

Main article: Null (SQL)

The concept of Null was introduced into SQL to handle missing information in the relational model. The word NULL is a reserved keyword in SQL, used to identify the Null special marker. Comparisons with Null, for instance equality (=) in WHERE clauses, results in an Unknown truth value. In SELECT statements SQL returns only results for which the WHERE clause returns a value of True; i.e. it excludes results with values of False and also excludes those whose value is Unknown.

Along with True and False, the Unknown resulting from direct comparisons with Null thus brings a fragment of three-valued logic to SQL. The truth tables SQL uses for AND, OR, and NOT correspond to a common fragment of the Kleene and Lukasiewicz three-valued logic (which differ in their definition of implication, however SQL defines no such operation).[19]

p AND qp
TrueFalseUnknown
qTrueTrueFalseUnknown
FalseFalseFalseFalse
UnknownUnknownFalseUnknown
p OR qp
TrueFalseUnknown
qTrueTrueTrueTrue
FalseTrueFalseUnknown
UnknownTrueUnknownUnknown
qNOT q
TrueFalse
FalseTrue
UnknownUnknown
p = qp
TrueFalseUnknown
qTrueTrueFalseUnknown
FalseFalseTrueUnknown
UnknownUnknownUnknownUnknown

There are however disputes about the semantic interpretation of Nulls in SQL because of its treatment outside direct comparisons. As seen in the table above direct equality comparisons between two NULLs in SQL (e.g. NULL = NULL) returns a truth value of Unknown. This is in line with the interpretation that Null does not have a value (and is not a member of any data domain) but is rather a placeholder or "mark" for missing information. However, the principle that two Nulls aren't equal to each other is effectively violated in the SQL specification for the UNION and INTERSECT operators, which do identify nulls with each other.[20] Consequently, these set operations in SQL may produce results not representing sure information, unlike operations involving explicit comparisons with NULL (e.g. those in a WHERE clause discussed above). In Codd's 1979 proposal (which was basically adopted by SQL92) this semantic inconsistency is rationalized by arguing that removal of duplicates in set operations happens "at a lower level of detail than equality testing in the evaluation of retrieval operations."[19] However, computer science professor Ron van der Meyden concluded that "The inconsistencies in the SQL standard mean that it is not possible to ascribe any intuitive logical semantics to the treatment of nulls in SQL."[20]

Additionally, since SQL operators return Unknown when comparing anything with Null directly, SQL provides two Null-specific comparison predicates: IS NULL and IS NOT NULL test whether data is or is not Null.[21] Universal quantification is not explicitly supported by SQL, and must be worked out as a negated existential quantification.[22][23][24] There is also the "<row value expression> IS DISTINCT FROM <row value expression>" infixed comparison operator, which returns TRUE unless both operands are equal or both are NULL. Likewise, IS NOT DISTINCT FROM is defined as "NOT (<row value expression> IS DISTINCT FROM <row value expression>)". SQL:1999 also introduced BOOLEAN type variables, which according to the standard can also hold Unknown values. In practice, a number of systems (e.g. PostgreSQL) implement the BOOLEAN Unknown as a BOOLEAN NULL.

Data manipulation[edit]

The Data Manipulation Language (DML) is the subset of SQL used to add, update and delete data:

 INSERT INTO example  (field1, field2, field3)  VALUES  ('test', 'N', NULL); 
 UPDATE example  SET field1 = 'updated value'  WHERE field2 = 'N'; 
 DELETE FROM example  WHERE field2 = 'N'; 
  MERGE INTO TABLE_NAME USING table_reference ON (condition)  WHEN MATCHED THEN  UPDATE SET column1 = value1 [, column2 = value2 ...]  WHEN NOT MATCHED THEN  INSERT (column1 [, column2 ...]) VALUES (value1 [, value2 ... 

Transaction controls[edit]

Transactions, if available, wrap DML operations:

 CREATE TABLE tbl_1(id INT);  INSERT INTO tbl_1(id) VALUES(1);  INSERT INTO tbl_1(id) VALUES(2); COMMIT;  UPDATE tbl_1 SET id=200 WHERE id=1; SAVEPOINT id_1upd;  UPDATE tbl_1 SET id=1000 WHERE id=2; ROLLBACK TO id_1upd;  SELECT id FROM tbl_1; 

COMMIT and ROLLBACK terminate the current transaction and release data locks. In the absence of a START TRANSACTION or similar statement, the semantics of SQL are implementation-dependent. The following example shows a classic transfer of funds transaction, where money is removed from one account and added to another. If either the removal or the addition fails, the entire transaction is rolled back.

 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; 

Data definition[edit]

The Data Definition Language (DDL) manages table and index structure. The most basic items of DDL are the CREATE, ALTER, RENAME, DROP and TRUNCATE statements:

 CREATE TABLE example(  field1 INTEGER,  field2 VARCHAR(50),  field3 DATE NOT NULL,  PRIMARY KEY (field1, field2) ); 
 ALTER TABLE example ADD field4 NUMBER(3) NOT NULL; 
 TRUNCATE TABLE example; 
 DROP TABLE example; 

Data types[edit]

Each column in an SQL table declares the type(s) that column may contain. ANSI SQL includes the following data types.[25]

Character strings[edit]

Bit strings[edit]

Numbers[edit]

For example, the number 123.45 has a precision of 5 and a scale of 2. The precision is a positive integer that determines the number of significant digits in a particular radix (binary or decimal). The scale is a non-negative integer. A scale of 0 indicates that the number is an integer. For a decimal number with scale S, the exact numeric value is the integer value of the significant digits divided by 10S.

SQL provides a function to round numerics or dates, called TRUNC (in Informix, DB2, PostgreSQL, Oracle and MySQL) or ROUND (in Informix, SQLite, Sybase, Oracle, PostgreSQL and Microsoft SQL Server)[26]

Date and time[edit]

SQL provides several functions for generating a date / time variable out of a date / time string (TO_DATE, TO_TIME, TO_TIMESTAMP), as well as for extracting the respective members (seconds, for instance) of such variables. The current system date / time of the database server can be called by using functions like NOW. The IBM Informix implementation provides the EXTEND and the FRACTION functions to increase the accuracy of time, for systems requiring sub-second precision.[27]

Data control[edit]

The Data Control Language (DCL) authorizes users to access and manipulate data. Its two main statements are:

Example:

 GRANT SELECT, UPDATE  ON example  TO some_user, another_user;   REVOKE SELECT, UPDATE  ON example  FROM some_user, another_user; 

Procedural extensions[edit]

SQL is designed for a specific purpose: to query data contained in a relational database. SQL is a set-based, declarative query language, not an imperative language like C or BASIC. However, extensions to Standard SQL add procedural programming language functionality, such as control-of-flow constructs. These include:

SourceCommon nameFull name
ANSI/ISO StandardSQL/PSMSQL/Persistent Stored Modules
Interbase / FirebirdPSQLProcedural SQL
IBM DB2SQL PLSQL Procedural Language (implements SQL/PSM)
IBM InformixSPLStored Procedural Language
IBM NetezzaNZPLSQL [1](based on Postgres PL/pgSQL)
Microsoft / SybaseT-SQLTransact-SQL
Mimer SQLSQL/PSMSQL/Persistent Stored Module (implements SQL/PSM)
MySQLSQL/PSMSQL/Persistent Stored Module (implements SQL/PSM)
NuoDBSSPStarkey Stored Procedures
OraclePL/SQLProcedural Language/SQL (based on Ada)
PostgreSQLPL/pgSQLProcedural Language/PostgreSQL (based on Oracle PL/SQL)
PostgreSQLPL/PSMProcedural Language/Persistent Stored Modules (implements SQL/PSM)
SybaseWatcom-SQLSQL Anywhere Watcom-SQL Dialect
TeradataSPLStored Procedural Language

In addition to the standard SQL/PSM extensions and proprietary SQL extensions, procedural and 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 code in SQL databases. SQL Server 2005 uses the SQLCLR (SQL Server Common Language Runtime) to host managed .NET assemblies in the database, while prior versions of SQL Server were restricted to unmanaged extended stored procedures primarily written in C. PostgreSQL lets users write functions in a wide variety of languages—including Perl, Python, Tcl, and C.[28]

Criticism[edit]

SQL deviates in several ways from its theoretical foundation, the relational model and its tuple calculus. In that model, a table is a set of tuples, while in SQL, tables and query results are lists of rows: the same row may occur multiple times, and the order of rows can be employed in queries (e.g. in the LIMIT clause). Whether this is a common practical concern, it is also a subject of debate. Furthermore, additional features (such as NULL and views) were introduced without founding them directly on the relational model, which makes them more difficult to interpret.

Critics argue that SQL should be replaced with a language that strictly returns to the original foundation: for example, see The Third Manifesto. Other critics suggest that Datalog has two advantages over SQL: it has cleaner semantics, which facilitates program understanding and maintenance, and it is more expressive, in particular for recursive queries.[29]

Another criticism is that SQL implementations are incompatible between vendors. In particular date and time syntax, string concatenation, NULLs, and comparison case sensitivity vary from vendor to vendor. A particular exception is PostgreSQL, which strives for compliance.[30]

Popular implementations of SQL commonly omit support for basic features of Standard SQL, such as the DATE or TIME data types. The most obvious such examples, and incidentally the most popular commercial and proprietary SQL DBMSs, are Oracle (whose DATE behaves as DATETIME,[31][32] and lacks a TIME type)[33] and MS SQL Server (before the 2008 version). 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:

Standardization[edit]

SQL was adopted as a standard by the American National Standards Institute (ANSI) in 1986 as SQL-86[34] and the International Organization for Standardization (ISO) in 1987. Nowadays the standard is subject to continuous improvement by the Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 32, Data management and interchange, which affiliate to ISO as well as IEC. It is commonly denoted by the pattern: ISO/IEC 9075-n:yyyy Part n: title, or, as a shortcut, ISO/IEC 9075.

ISO/IEC 9075 is complemented by ISO/IEC 13249: SQL Multimedia and Application Packages (SQL/MM), which defines SQL based interfaces and packages to widely spread applications like video, audio and spatial data.

Until 1996, the National Institute of Standards and Technology (NIST) data management standards program certified SQL DBMS compliance with the SQL standard. Vendors now self-certify the compliance of their products.[35]

The original standard declared that the official pronunciation for "SQL" was an initialism: /ˈɛs kjuː ˈɛl/ ("es queue el").[10] Regardless, many English-speaking database professionals (including Donald Chamberlin himself[36]) use the acronym-like pronunciation of /ˈskwəl/ ("sequel"),[37] mirroring the language's pre-release development name of "SEQUEL".[13][14]

The SQL standard has gone through a number of revisions:

YearNameAliasComments
1986SQL-86SQL-87First formalized by ANSI.
1989SQL-89FIPS 127-1Minor revision, in which the major addition were integrity constraints. Adopted as FIPS 127-1.
1992SQL-92SQL2, FIPS 127-2Major revision (ISO 9075), Entry Level SQL-92 adopted as FIPS 127-2.
1999SQL:1999SQL3Added regular expression matching, recursive queries (e.g. transitive closure), triggers, support for procedural and control-of-flow statements, non-scalar types, and some object-oriented features (e.g. structured types). Support for embedding SQL in Java (SQL/OLB) and vice-versa (SQL/JRT).
2003SQL:2003SQL 2003Introduced XML-related features (SQL/XML), window functions, standardized sequences, and columns with auto-generated values (including identity-columns).
2006SQL:2006SQL 2006ISO/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 enables 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.[38]
2008SQL:2008SQL 2008Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers. Adds the TRUNCATE statement.[39]
2011SQL:2011

Interested parties may purchase SQL standards documents from ISO,[40] IEC or ANSI. A draft of SQL:2008 is freely available as a zip archive.[41]

The SQL standard is divided into nine parts.

ISO/IEC 9075 is complemented by ISO/IEC 13249 SQL Multimedia and Application Packages. This closely related but separate standard is developed by the same committee. It defines interfaces and packages based on SQL. The aim is a unified access to typical database applications like text, pictures, data mining or spatial data.

Alternatives[edit]

A distinction should be made between alternatives to SQL as a language, and alternatives to the relational model itself. Below are proposed relational alternatives to the SQL language. See navigational database and NoSQL for alternatives to the relational model.

See also[edit]

Notes[edit]

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  2. ^ "Media Type registration for application/sql". Internet Assigned Numbers Authority. 10 April 2013. Retrieved 10 April 2013. 
  3. ^ "The application/sql Media Type, RFC 6922". Internet Engineering Task Force. April 2013. p. 3. Retrieved 10 April 2013. 
  4. ^ Beaulieu, Alan (April 2009). Mary E Treseler, ed. Learning SQL (2nd ed.). Sebastapol, CA, USA: O'Reilly. ISBN 978-0-596-52083-0. 
  5. ^ Encyclopedia Britannica. "SQL". Retrieved 2013-04-02. 
  6. ^ From Oxford Dictionaries: "Definition of SQL - abbreviation, Structured Query Language, an international standard for database manipulation." Oxford Dictionaries. "SQL". 
  7. ^ IBM. "SQL Guide". 
  8. ^ From Microsoft: "Structured Query Language, invented at IBM in the 1970s. It is more commonly known by its acronym, SQL .." Microsoft. "Structured Query Language (SQL)". 
  9. ^ Codd, Edgar F (June 1970). "A Relational Model of Data for Large Shared Data Banks". Communications of the ACM (Association for Computing Machinery) 13 (6): 377–87. doi:10.1145/362384.362685. Retrieved 2007-06-09. 
  10. ^ a b Chapple, Mike. "SQL Fundamentals". Databases. About.com. Retrieved 2009-01-28. 
  11. ^ "Structured Query Language (SQL)". International Business Machines. October 27, 2006. Retrieved 2007-06-10. 
  12. ^ "ISO/IEC 9075-1:2008: Information technology – Database languages – SQL – Part 1: Framework (SQL/Framework)". 
  13. ^ a b c Chamberlin, Donald D; Boyce, Raymond F (1974). "SEQUEL: A Structured English Query Language" (PDF). Proceedings of the 1974 ACM SIGFIDET Workshop on Data Description, Access and Control (Association for Computing Machinery): 249–64. Retrieved 2007-06-09. 
  14. ^ a b Oppel, Andy (February 27, 2004). Databases Demystified. San Francisco, CA: McGraw-Hill Osborne Media. pp. 90–1. ISBN 0-07-146960-5. 
  15. ^ "History of IBM, 1978". IBM Archives. IBM. Retrieved 2007-06-09. 
  16. ^ ANSI/ISO/IEC International Standard (IS). Database Language SQL—Part 2: Foundation (SQL/Foundation). 1999.
  17. ^ "DECODE". Docs.oracle.com. Retrieved 2013-06-14. 
  18. ^ "Transact-SQL Reference". SQL Server Language Reference. SQL Server 2005 Books Online. Microsoft. 2007-09-15. Retrieved 2007-06-17. 
  19. ^ a b Hans-Joachim, K. (2003). "Null Values in Relational Databases and Sure Information Answers". Semantics in Databases. Second International Workshop Dagstuhl Castle, Germany, January 7–12, 2001. Revised Papers. Lecture Notes in Computer Science 2582. pp. 119–138. doi:10.1007/3-540-36596-6_7. ISBN 978-3-540-00957-3.  edit
  20. ^ a b Ron van der Meyden, "Logical approaches to incomplete information: a survey" in Chomicki, Jan; Saake, Gunter (Eds.) Logics for Databases and Information Systems, Kluwer Academic Publishers ISBN 978-0-7923-8129-7, p. 344; PS preprint (note: page numbering differs in preprint from the published version)
  21. ^ ISO/IEC. ISO/IEC 9075-2:2003, "SQL/Foundation". ISO/IEC. 
  22. ^ M. Negri, G. Pelagatti, L. Sbattella (1989) Semantics and problems of universal quantification in SQL.
  23. ^ Fratarcangeli, Claudio (1991). Technique for universal quantification in SQL. Retrieved from ACM.org.
  24. ^ Kawash, Jalal (2004) 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, Virginia. Retrieved from Thefreelibrary.com.
  25. ^ Information Technology: Database Language SQL. CMU.  (proposed revised text of DIS 9075).
  26. ^ Arie Jones, Ryan K. Stephens, Ronald R. Plew, Alex Kriegel, Robert F. Garrett (2005), SQL Functions Programmer's Reference. Wiley, 127 pages.
  27. ^ http://publib.boulder.ibm.com/infocenter/idshelp/v111/index.jsp?topic=/com.ibm.sqlr.doc/sqlr150.htm
  28. ^ PostgreSQL contributors (2011). "PostgreSQL server programming". PostgreSQL 9.1 official documentation. postgresql.org. Retrieved 2012-03-09. 
  29. ^ http://lbd.udc.es/jornadas2011/actas/PROLE/PROLE/S5/13_article.pdf
  30. ^ PostgreSQL contributors (2012). "About PostgreSQL". PostgreSQL 9.1 official website. PostgreSQL Global Development Group. Retrieved March 9, 2012. "PostgreSQL prides itself in standards compliance. Its SQL implementation strongly conforms to the ANSI-SQL:2008 standard" 
  31. ^ Lorentz, Diana; Roeser, Mary Beth; Abraham, Sundeep; Amor, Angela; Arora, Geeta; Arora, Vikas; Ashdown, Lance; Baer, Hermann; Bellamkonda, Shrikanth (October 2010) [1996]. "Basic Elements of Oracle SQL: Data Types". Oracle Database SQL Language Reference 11g Release 2 (11.2). Oracle Database Documentation Library. Redwood City, CA: Oracle USA, Inc. Retrieved December 29, 2010. "For each DATE value, Oracle stores the following information: century, year, month, date, hour, minute, and second" 
  32. ^ Lorentz, Diana; Roeser, Mary Beth; Abraham, Sundeep; Amor, Angela; Arora, Geeta; Arora, Vikas; Ashdown, Lance; Baer, Hermann; Bellamkonda, Shrikanth (October 2010) [1996]. "Basic Elements of Oracle SQL: Data Types". Oracle Database SQL Language Reference 11g Release 2 (11.2). Oracle Database Documentation Library. Redwood City, CA: Oracle USA, Inc. Retrieved December 29, 2010. "The datetime data types are DATE..." 
  33. ^ Lorentz, Diana; Roeser, Mary Beth; Abraham, Sundeep; Amor, Angela; Arora, Geeta; Arora, Vikas; Ashdown, Lance; Baer, Hermann; Bellamkonda, Shrikanth (October 2010) [1996]. "Basic Elements of Oracle SQL: Data Types". Oracle Database SQL Language Reference 11g Release 2 (11.2). Oracle Database Documentation Library. Redwood City, CA: Oracle USA, Inc. Retrieved December 29, 2010. "Do not define columns with the following SQL/DS and DB2 data types, because they have no corresponding Oracle data type:... TIME" 
  34. ^ "Finding Aid". X3H2 Records, 1978–95. American National Standards Institute. 
  35. ^ Doll, Shelley (June 19, 2002). "Is SQL a Standard Anymore?". TechRepublic's Builder.com. TechRepublic. Archived from the original on 2013-01-02. Retrieved 2010-01-07. 
  36. ^ Gillespie, Patrick. "Pronouncing SQL: S-Q-L or Sequel?". Pronouncing SQL: S-Q-L or Sequel?. Retrieved 12 February 2012. 
  37. ^ Melton, Jim; Alan R Simon (1993). "1.2. What is SQL?". Understanding the New SQL: A Complete Guide. Morgan Kaufmann. p. 536. ISBN 1-55860-245-3. "SQL (correctly pronounced "ess cue ell," instead of the somewhat common "sequel")..." 
  38. ^ a b Wagner, Michael (2010). SQL/XML:2006 - Evaluierung der Standardkonformität ausgewählter Datenbanksysteme. Diplomica Verlag. p. 100. ISBN 3-8366-9609-6. 
  39. ^ SQL:2008 now an approved ISO international standard. Sybase. July 2008. 
  40. ^ "ISO/IEC 9075-2:2011: Information technology -- Database languages -- SQL -- Part 2: Foundation (SQL/Foundation)". 
  41. ^ SQL:2008 draft (Zip). Whitemarsh Information Systems Corporation. 
  42. ^ ISO/IEC 9075-11:2008: Information and Definition Schemas (SQL/Schemata). 2008. p. 1. 

References[edit]

External links[edit]