|This article's lead section may not adequately summarize key points of its contents. (June 2015)|
||This article may be in need of reorganization to comply with Wikipedia's layout guidelines. (June 2015) (Learn how and when to remove this template message)|
|Designed by||Donald D. Chamberlin
Raymond F. Boyce
|Typing discipline||Static, strong|
|File formats||File format details|
|Internet media type||
|Type of format||Database|
|CQL, LINQ, SOQL, PowerShell,JPQL, jOOQ|
SQL ( or ,Structured Query Language) 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." Despite not entirely adhering to the relational model as described by Codd, it became the most widely used database language.
SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987. 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.
SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce in the early 1970s. 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. The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company.
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.
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.
||It has been suggested that this section be split out into another article titled SQL syntax. (Discuss) (June 2015)|
The SQL language is subdivided into several language elements, including:
||Not equal to (many DBMSs accept
||Greater than or equal||
||Less than or equal||
||Between an inclusive range||
||Match a character pattern||
||Equal to one of multiple possible values||
||Compare to null (missing data)||
||Is equal to value or both are nulls (missing data)||
||Used to change a field name when viewing results||
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, it works like else if in other programming languages:
CASE WHEN n > 0 THEN 'positive' WHEN n < 0 THEN 'negative' ELSE 'zero' END
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
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.
The most common operation in SQL, the query, makes use of the declarative
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.
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:
FROMclause, which indicates the table(s) to retrieve data from. The
FROMclause can include optional
JOINsubclauses to specify the rules for joining tables.
WHEREclause includes a comparison predicate, which restricts the rows returned by the query. The
WHEREclause eliminates all rows from the result set where the comparison predicate does not evaluate to True.
GROUP BYclause projects rows having common values into a smaller set of rows.
GROUP BYis often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The
WHEREclause is applied before the
HAVINGclause includes a predicate used to filter rows resulting from the
GROUP BYclause. Because it acts on the results of the
GROUP BYclause, aggregation functions can be used in the
ORDER BYclause identifies which column[s] to use to sort the resulting data, and in which direction to sort them (ascending or descending). Without an
ORDER BYclause, the order of rows returned by an SQL query is undefined.
DISTINCTkeyword eliminates duplicate data.
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;
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 expressions (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.
An Inline view is the use of referencing an SQL subquery in a FROM clause. Essentially, the inline view is a subquery that can be selected from or joined to. The Inline view functionality allows the user to reference the subquery as a table. The inline view is also referred to as a derived table or a subselect. It was introduced in Oracle 9i.
In the following example, the SQL statement involves a join from the initial "Book" table to the Inline view "Sales". This Inline view captures associated book sales information using the ISBN to join to the "Book" table. As a result, the Inline view 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
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).
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
INTERSECT operators, which do identify nulls with each other. 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". 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."
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. SQL does not explicitly support universal quantification, and must work it out as a negated existential quantification. 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) [clarify].
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);
UPDATEmodifies a set of existing table rows, e.g.:
UPDATE example SET field1 = 'updated value' WHERE field2 = 'N';
DELETEremoves existing rows from a table, e.g.:
DELETE FROM example WHERE field2 = 'N';
MERGEis used to combine the data of multiple tables. It combines the
UPDATEelements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called "upsert".
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 ...])
Transactions, if available, wrap DML operations:
BEGIN WORK, or
BEGIN TRANSACTION, depending on SQL dialect) marks the start of a database transaction, which either completes entirely or not at all.
SAVEPOINT) saves the state of the database at the current point in transaction
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;
COMMITmakes all data changes in a transaction permanent.
ROLLBACKdiscards all data changes since the last
ROLLBACK, leaving the data as it was prior to those changes. Once the
COMMITstatement completes, the transaction's changes cannot be rolled back.
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;
The Data Definition Language (DDL) manages table and index structure. The most basic items of DDL are the
CREATEcreates an object (a table, for example) in the database, e.g.:
CREATE TABLE example( column1 INTEGER, column2 VARCHAR(50), column3 DATE NOT NULL, PRIMARY KEY (column1, column2) );
ALTERmodifies the structure of an existing object in various ways, for example, adding a column to an existing table or a constraint, e.g.:
ALTER TABLE example ADD column4 NUMBER(3) NOT NULL;
TRUNCATEdeletes all data from a table in a very fast way, deleting the data inside the table and not the table itself. It usually implies a subsequent COMMIT operation, i.e., it cannot be rolled back (data is not written to the logs for rollback later, unlike DELETE).
TRUNCATE TABLE example;
DROPdeletes an object in the database, usually irretrievably, i.e., it cannot be rolled back, e.g.:
DROP TABLE example;
Each column in an SQL table declares the type(s) that column may contain. ANSI SQL includes the following data types.
CHAR(n): fixed-width n-character string, padded with spaces as needed
VARCHAR(n): variable-width string with a maximum size of n characters
NCHAR(n): fixed width string supporting an international character set
NATIONAL CHARACTER VARYING(n)or
BIT(n): an array of n bits
BIT VARYING(n): an array of up to n bits
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)
DATE: for date values (e.g.
TIME: for time values (e.g.
15:51:36). The granularity of the time value is usually a tick (100 nanoseconds).
TIME WITH TIME ZONEor
TIMETZ: the same as
TIME, but including details about the time zone in question.
TIMESTAMP: This is a
TIMEput together in one variable (e.g.
TIMESTAMP WITH TIME ZONEor
TIMESTAMPTZ: the same as
TIMESTAMP, but including details about the time zone in question.
SQL provides several functions for generating a date / time variable out of a date / time string (
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.
The Data Control Language (DCL) authorizes users to access and manipulate data. Its two main statements are:
GRANTauthorizes one or more users to perform an operation or a set of operations on an object.
REVOKEeliminates a grant, which may be the default grant.
GRANT SELECT, UPDATE ON example TO some_user, another_user; REVOKE SELECT, UPDATE ON example FROM some_user, another_user;
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 ||(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, and C.
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. A particular exception is PostgreSQL, which strives for standards compliance.
Popular implementations of SQL commonly omit support for basic features of Standard SQL, such as the
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, and lacks a
TIME type) 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 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.
The original standard declared that the official pronunciation for "SQL" was an initialism: ("es queue el"). Regardless, many English-speaking database professionals (including Donald Chamberlin himself) use the acronym-like pronunciation of ("sequel"), mirroring the language's pre-release development name of "SEQUEL". The SQL standard has gone through a number of revisions:
|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.|
|2008||SQL:2008||Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers. Adds the TRUNCATE statement.|
|2011||SQL:2011||Adds temporal data definition and manipulation.|
|2016||SQL:2016||Adds row pattern matching, polymorphic table functions, JSON.|
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.
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 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.
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.
Although the UNIQUE argument is identical to DISTINCT, it is not an ANSI standard.
[...] the keyword DISTINCT [...] eliminates the duplicates from the result set.
PostgreSQL prides itself in standards compliance. Its SQL implementation strongly conforms to the ANSI-SQL:2008 standard
DATEvalue, Oracle stores the following information: century, year, month, date, hour, minute, and second
The datetime data types are
Do not define columns with the following SQL/DS and DB2 data types, because they have no corresponding Oracle data type:...
SQL (correctly pronounced "ess cue ell," instead of the somewhat common "sequel")...
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