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SQL
"Paradigm "Multi-paradigm: "declarative
Family "Programming language
"Designed by "Donald D. Chamberlin
"Raymond F. Boyce
"Developer "ISO/"IEC
First appeared 1974; 43 years ago (1974)
"Typing discipline "Static, "strong
"OS "Cross-platform
"File formats File format details
"Filename extension .sql
"Internet media type application/sql[1][2]
Developed by "ISO/"IEC
Initial release 1986 (1986)
"Latest release
"SQL:2016
(December 2016; 5 months ago (2016-12))
Type of format Database
"Standard ISO/IEC 9075
"Open format? Yes
Major "implementations
"Many
"Dialects
Influenced by
"Datalog
Influenced
"CQL, "LINQ, SOQL, "PowerShell,[3] "JPQL, "jOOQ, "N1QL

SQL (Listen""i"/ˈɛs kjuː ˈɛl/[4] or Listen""i"/ˈskwəl/,[5] Structured Query Language[6][7][8][9]) is a "domain-specific language used in programming and designed for managing data held in a "relational database management system (RDBMS), or for stream processing in a "relational data stream management system (RDSMS).

Originally based upon "relational algebra and "tuple relational calculus, SQL consists of a "data definition language, "data manipulation language, and "data control 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."[10] Despite not entirely adhering to "the relational model as described by Codd, it became the most widely used database language.[11][12]

SQL became a "standard of the "American National Standards Institute (ANSI) in 1986, and of the "International Organization for Standardization (ISO) in 1987.[13] Since then, the standard has been revised to include a larger set of features. Despite the existence of such standards, most SQL code is not completely portable among different database systems without adjustments.

Contents

History[edit]

SQL was initially developed at "IBM by "Donald D. Chamberlin and "Raymond F. Boyce in the early 1970s.[14] 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.[14] The acronym SEQUEL was later changed to SQL because "SEQUEL" was a "trademark of the "UK-based "Hawker Siddeley aircraft company.[15]

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.[16]

Design[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).

Critics argue that SQL should be replaced with a language that strictly returns to the original foundation: for example, see "The Third Manifesto.

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]

Operator Description Example
= Equal to Author = 'Alcott'
<> Not equal to (many DBMSs accept != in addition to <>) Dept <> 'Sales'
> Greater than Hire_Date > '2012-01-31'
< Less than Bonus < 50000.00
>= Greater than or equal Dependents >= 2
<= Less than or equal Rate <= 0.05
BETWEEN Between an inclusive range Cost BETWEEN 100.00 AND 500.00
"LIKE Match a character pattern First_Name LIKE 'Will%'
IN Equal to one of multiple possible values DeptCode IN (101, 103, 209)
IS or IS NOT Compare to null (missing data) Address IS NOT NULL
IS NOT DISTINCT FROM Is equal to value or both are nulls (missing data) Debt IS NOT DISTINCT FROM - Receivables
AS Used to change a column name when viewing results SELECT employee AS "department1"

Other operators have at times been suggested or implemented, such as the "skyline operator (for finding only those records that are not 'worse' than any others).

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:

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.

There are two short forms for special CASE expressions: COALESCE and NULLIF.

The COALESCE expression returns the value of the first non-NULL operand, found by working from left to right, or NULL if all the operands equal NULL.

COALESCE(x1,x2)

is equivalent to:

CASE WHEN x1 IS NOT NULL THEN x1
     ELSE x2
END

The NULLIF expression has two operands and returns NULL if the operands have the same value, otherwise it has the value of the first operand.

NULLIF(x1, x2)

is equivalent to

CASE WHEN x1 = x2 THEN NULL ELSE x1 END

Queries[edit]

The most common operation in SQL, the query, makes use of 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 provided in some databases.[18]

Queries allow the user to describe desired data, leaving the "database management system (DBMS) to carry out "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, normally immediately following the SELECT keyword. An asterisk ("*") can 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 example of a SELECT query 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 Book table, one could re-write the query above in the following form:

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

However, many["quantify] 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 WITH clauses for subqueries, i.e. named subqueries, usually called "common table expressions (also called "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.

Derived table[edit]

A derived table is the use of referencing an SQL subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. The derived table functionality allows the user to reference the subquery as a table. The inline view is also referred to as an inline view or a subselect.

In the following example, the SQL statement involves a join from the initial "Book" table to the derived table "sales". This derived table captures associated book sales information using the ISBN to join to the "Book" table. As a result, the derived table provides the result set with additional columns (the number of items sold and the company that sold the books):

SELECT b.isbn, b.title, b.price, sales.items_sold, sales.company_nm
FROM Book b
  JOIN (SELECT SUM(Items_Sold) Items_Sold, Company_Nm, ISBN
        FROM Book_Sales
        GROUP BY Company_Nm, ISBN) sales
  ON sales.isbn = b.isbn

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

The concept of "Null allows SQL to deal with 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).[21]

p AND q p
True False Unknown
q True True False Unknown
False False False False
Unknown Unknown False Unknown
p OR q p
True False Unknown
q True True True True
False True False Unknown
Unknown True Unknown Unknown
p = q p
True False Unknown
q True True False Unknown
False False True Unknown
Unknown Unknown Unknown Unknown
q NOT q
True False
False True
Unknown Unknown

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) return 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.[22] 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".[21] 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."[22]

Additionally, because 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.[23] SQL does not explicitly support "universal quantification, and must work it out as a negated "existential quantification.[24][25][26] 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["clarify].

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(
 column1 INTEGER,
 column2 VARCHAR(50),
 column3 DATE NOT NULL,
 PRIMARY KEY (column1, column2)
);
ALTER TABLE example ADD column4 INTEGER 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.[13]

Character strings and national character strings

For the CHARACTER LARGE OBJECT and NATIONAL CHARACTER LARGE OBJECT data types, the multipliers K (1 024), M (1 048 576), G (1 073 741 824) and T (1 099 511 627 776) can be optionally used when specifying the length.

Binary

For the BINARY LARGE OBJECT data type, the multipliers K (1 024), M (1 048 576), G (1 073 741 824) and T (1 099 511 627 776) can be optionally used when specifying the length.

Boolean

The BOOLEAN data type can store the values TRUE and FALSE.

Numerical

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 function the functions CEILING and FLOOR to round numerical values. (Popular vendor specific functions are TRUNC (Informix, DB2, PostgreSQL, Oracle and MySQL) and ROUND (Informix, SQLite, Sybase, Oracle, PostgreSQL, Microsoft SQL Server and Mimer SQL.))

Temporal (datetime)

The SQL function EXTRACT can be used for extracting a single field (seconds, for instance) of a datetime or interval value. The current system date / time of the database server can be called by using functions like CURRENT_DATE, CURRENT_TIMESTAMP, LOCALTIME, or LOCALTIMESTAMP. (Popular vendor specific functions are TO_DATE, TO_TIME, TO_TIMESTAMP, YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, DAYOFYEAR, DAYOFMONTH and DAYOFWEEK.)

Interval (datetime)

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 programming language, not an "imperative programming language like "C or "BASIC. However, extensions to Standard SQL add "procedural programming language functionality, such as control-of-flow constructs. These include:

Source Common name Full name
ANSI/ISO Standard "SQL/PSM SQL/Persistent Stored Modules
"Interbase / "Firebird "PSQL Procedural SQL
"IBM DB2 "SQL PL SQL Procedural Language (implements SQL/PSM)
"IBM Informix SPL Stored Procedural Language
IBM "Netezza NZPLSQL [1] (based on Postgres PL/pgSQL)
"Microsoft / "Sybase "T-SQL Transact-SQL
"Mimer SQL "SQL/PSM SQL/Persistent Stored Module (implements SQL/PSM)
"MySQL "SQL/PSM SQL/Persistent Stored Module (implements SQL/PSM)
"MonetDB "SQL/PSM SQL/Persistent Stored Module (implements SQL/PSM)
"NuoDB SSP Starkey Stored Procedures
"Oracle "PL/SQL Procedural Language/SQL (based on "Ada)
"PostgreSQL "PL/pgSQL Procedural Language/PostgreSQL Structured Query Language (implements SQL/PSM)
"Sybase Watcom-SQL SQL Anywhere Watcom-SQL Dialect
"Teradata SPL Stored Procedural Language
"SAP "SAP HANA SQL Script

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, "JavaScript (PL/V8) and C.[27]

Interoperability and standardization[edit]

SQL implementations are incompatible between vendors and do not necessarily completely follow standards. In particular date and time syntax, string concatenation, NULLs, and comparison "case sensitivity vary from vendor to vendor. Particular exceptions are "PostgreSQL[28] and "Mimer SQL[29] who strive for standards compliance.

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,[30][31] and lacks a TIME type)[32] 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:

SQL was adopted as a standard by the "American National Standards Institute (ANSI) in 1986 as SQL-86[33] 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.[34]

The original standard declared that the official pronunciation for "SQL" was an "initialism: "/ˈɛs kjuː ˈɛl/ ("es queue el").[11] Regardless, many English-speaking database professionals (including "Donald Chamberlin himself[5]) use the "acronym-like pronunciation of "/ˈskwəl/ ("sequel"),[35] mirroring the language's pre-release development name of "SEQUEL".[14][15] [5][14] The SQL standard has gone through a number of revisions:

Year Name Alias Comments
1986 SQL-86 SQL-87 First formalized by ANSI.
1989 SQL-89 "FIPS 127-1 Minor revision that added integrity constraints, adopted as FIPS 127-1.
1992 "SQL-92 SQL2, FIPS 127-2 Major revision (ISO 9075), Entry Level SQL-92 adopted as FIPS 127-2.
1999 "SQL:1999 SQL3 Added 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).
2003 "SQL:2003 Introduced "XML-related features ("SQL/XML), window functions, standardized sequences, and columns with auto-generated values (including identity-columns).
2006 "SQL:2006 ISO/IEC 9075-14:2006 defines ways that SQL can be used 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 lets applications integrate queries into their SQL code with "XQuery, the XML Query Language published by the World Wide Web Consortium ("W3C), to concurrently access ordinary SQL-data and XML documents.[36]
2008 "SQL:2008 Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers. Adds the TRUNCATE statement.[37]
2011 "SQL:2011 Adds temporal data definition and manipulation.
2016 "SQL:2016 Adds row pattern matching, polymorphic table functions, JSON.

Interested parties may purchase SQL standards documents from ISO,[38] IEC or ANSI. A draft of SQL:2008 is freely available as a "zip archive.[39]

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.

Distributed SQL processing[edit]

"Distributed Relational Database Architecture (DRDA) was designed by a work group within IBM in the period 1988 to 1994. DRDA enables network connected relational databases to cooperate to fulfill SQL requests.[42][43]

An interactive user or program can issue SQL statements to a local RDB and receive tables of data and status indicators in reply from remote RDBs. SQL statements can also be compiled and stored in remote RDBs as packages and then invoked by package name. This is important for the efficient operation of application programs that issue complex, high-frequency queries. It is especially important when the tables to be accessed are located in remote systems.

The messages, protocols, and structural components of DRDA are defined by the "Distributed Data Management Architecture.

See also[edit]

Notes[edit]

  1. ^ "Media Type registration for application/sql". "Internet Assigned Numbers Authority. 10 April 2013. Retrieved 10 April 2013. 
  2. ^ "The application/sql Media Type, RFC 6922". "Internet Engineering Task Force. April 2013. p. 3. Retrieved 10 April 2013. 
  3. ^ Paul, Ryan. "A guided tour of the Microsoft Command Shell". Ars Technica. Retrieved 10 April 2011. 
  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. ^ a b c Gillespie, Patrick. "Pronouncing SQL: S-Q-L or Sequel?". Pronouncing SQL: S-Q-L or Sequel?. Retrieved 12 February 2012. 
  6. ^ "SQL". Britannica.com. Retrieved 2013-04-02. 
  7. ^ "SQL". Oxforddictionaries.com. Retrieved 2017-01-16. 
  8. ^ "SQL Guide". Publib.boulder.ibm.com. Retrieved 2017-01-16. 
  9. ^ "Structured Query Language (SQL)". Msdn.microsoft.com. Retrieved 2017-01-16. 
  10. ^ 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. 
  11. ^ a b Chapple, Mike. "SQL Fundamentals". Databases. About.com. Retrieved 2009-01-28. 
  12. ^ "Structured Query Language (SQL)". International Business Machines. October 27, 2006. Retrieved 2007-06-10. 
  13. ^ a b "ISO/IEC 9075-1:2016: Information technology – Database languages – SQL – Part 1: Framework (SQL/Framework)". 
  14. ^ a b c d 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. 
  15. ^ a b Oppel, Andy (February 27, 2004). Databases Demystified. "San Francisco, CA: McGraw-Hill Osborne Media. pp. 90–1. "ISBN "0-07-146960-5. 
  16. ^ "History of IBM, 1978". IBM Archives. IBM. Retrieved 2007-06-09. 
  17. ^ ANSI/ISO/IEC International Standard (IS). Database Language SQL—Part 2: Foundation (SQL/Foundation). 1999.
  18. ^ "Transact-SQL Reference". SQL Server Language Reference. SQL Server 2005 Books Online. Microsoft. 2007-09-15. Retrieved 2007-06-17. 
  19. ^ SAS 9.4 SQL Procedure User's Guide. SAS Institute. 2013. p. 248. "ISBN "9781612905686. Retrieved 2015-10-21. Although the UNIQUE argument is identical to DISTINCT, it is not an ANSI standard. 
  20. ^ "Leon, Alexis; Leon, Mathews (1999). "Eliminating duplicates - SELECT using DISTINCT". SQL: A Complete Reference. New Delhi: Tata McGraw-Hill Education (published 2008). p. 143. "ISBN "9780074637081. Retrieved 2015-10-21. [...] the keyword DISTINCT [...] eliminates the duplicates from the result set. 
  21. ^ 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. 
  22. ^ 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
  23. ^ ISO/IEC. ISO/IEC 9075-2:2003, "SQL/Foundation". ISO/IEC. 
  24. ^ "Semantics and problems of universal quantification in SQL". Portal.acm.org. "doi:10.1093/comjnl/32.1.90. Retrieved 2017-01-16. 
  25. ^ "Technique for universal quantification in SQL". Portal.acm.org. "doi:10.1145/126482.126484. Retrieved 2017-01-16. 
  26. ^ 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. Thefreelibrary.com
  27. ^ PostgreSQL contributors (2011). "PostgreSQL server programming". PostgreSQL 9.1 official documentation. postgresql.org. Retrieved 2012-03-09. 
  28. ^ 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 
  29. ^ "Mimer SQL, Built on Standards". Mimer SQL official website. Mimer Information Technology. 2009. 
  30. ^ 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 
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