SQL

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SQL
Paradigm:
multi-paradigm: object-oriented, functional, procedural
Appeared in:
1974
Designed by:
Donald D. Chamberlin and Raymond F. Boyce
Developer:
IBM
Typing discipline:
static, strong
Major implementations:
Many
SQL (commonly expanded to Structured Query Language — see History for the term's derivation) is the most popular computer language used to create, modify, retrieve and manipulate data from relational database management systems. The language has evolved beyond its original purpose to support object-relational database management systems. It is an ANSI/ISO standard.
SQL is commonly spoken either as the names of the letters ess-cue-el (IPA: [ˈɛsˈkjuˈɛl]), or like the word sequel (IPA: [ˈsiːkwəl]). Concerning the names of major database products (or projects) containing the letters SQL, each has its own convention: MySQL is officially and commonly pronounced "My Ess Cue El"; PostgreSQL is expediently pronounced postgres (which had been the predecessor to PostgreSQL); and Microsoft SQL Server is commonly spoken as Microsoft-sequel-server.
Contents[hide]
1 History
1.1 Standardization
2 Scope
2.1 Reasons for lack of portability
3 SQL keywords
3.1 Data retrieval
3.2 Data manipulation
3.3 Transaction Controls
3.4 Data definition
3.5 Data control
3.6 Other
4 Criticisms of SQL
5 Alternatives to SQL
6 See also
6.1 Database systems using SQL
6.2 SQL variants
7 References
8 External links
//

[edit] History
An influential paper, "A Relational Model of Data for Large Shared Data Banks", by Dr. Edgar F. Codd, was published in June, 1970 in the Association for Computing Machinery (ACM) journal, Communications of the ACM, although drafts of it were circulated internally within IBM in 1969. Codd's model became widely accepted as the definitive model for relational database management systems (RDBMS or RDMS).
During the 1970s, a group at IBM's San Jose research center developed a database system "System R" based upon, but not strictly faithful to, Codd's model. Structured English Query Language ("SEQUEL") was designed to manipulate and retrieve data stored in System R. The acronym SEQUEL was later condensed to SQL because the word 'SEQUEL' was held as a trademark by the Hawker-Siddeley aircraft company of the UK. Although SQL was influenced by Codd's work, Donald D. Chamberlin and Raymond F. Boyce at IBM were the authors of the SEQUEL language design.[1] Their concepts were published to increase interest in SQL.
The first non-commercial, relational, non-SQL database, Ingres, was developed in 1974 at U.C. Berkeley.
In 1978, methodical testing commenced at customer test sites. Demonstrating both the usefulness and practicality of the system, this testing proved to be a success for IBM. As a result, IBM began to develop commercial products based on their System R prototype that implemented SQL, including the System/38 (announced in 1978 and commercially available in August 1979), SQL/DS (introduced in 1981), and DB2 (in 1983).[1]
At the same time Relational Software, Inc. (now Oracle Corporation) saw the potential of the concepts described by Chamberlin and Boyce and developed their own version of a RDBMS for the Navy, CIA and others. In the summer of 1979 Relational Software, Inc. introduced Oracle V2 (Version2) for VAX computers as the first commercially available implementation of SQL. Oracle is often incorrectly cited as beating IBM to market by two years, when in fact they only beat IBM's release of the System/38 by a few weeks. Considerable public interest then developed; soon many other vendors developed versions, and Oracle's future was ensured.

[edit] Standardization
SQL was adopted as a standard by ANSI (American National Standards Institute) in 1986 and ISO (International Organization for Standardization) in 1987. ANSI has declared that the official pronunciation for SQL is /ɛs kjuː ɛl/, although many English-speaking database professionals still pronounce it as sequel.[citation needed]
The SQL standard has gone through a number of revisions:
Year
Name
Alias
Comments
1986
SQL-86
SQL-87
First published by ANSI. Ratified by ISO in 1987.
1989
SQL-89
Minor revision.
1992
SQL-92
SQL2
Major revision (ISO 9075).
1999
SQL:1999
SQL3
Added regular expression matching, recursive queries, triggers, non-scalar types and some object-oriented features. (The last two are somewhat controversial and not yet widely supported.)
2003
SQL:2003

Introduced XML-related features, window functions, standardized sequences and columns with auto-generated values (including identity-columns).
The SQL standard is not freely available. SQL:2003 may be purchased from ISO or ANSI. A late draft is available as a zip archive from Whitemarsh Information Systems Corporation. The zip archive contains a number of PDF files that define the parts of the SQL:2003 specification.

[edit] Scope

The neutrality of this article or section may be compromised by weasel words.You can help Wikipedia by improving weasel-worded statements.
SQL is defined by both ANSI and ISO.
Extensions to and variations of the standards exist: Oracle Corporation's PL/SQL, IBM's SQL PL (SQL Procedural Language) and Sybase / Microsoft's Transact-SQL, which are of a proprietary nature. Commercial implementations commonly omit support for basic features of the standard, such as the DATE or TIME data types, preferring variations of their own. SQL code can rarely be ported between database systems without major modifications, in contrast to ANSI C or ANSI Fortran, which can usually be ported from platform to platform without major structural changes.
SQL is designed for a specific, limited purpose — querying data contained in a relational database. As such, it is a set-based, declarative computer language rather than an imperative language such as C or BASIC which, being general-purpose, are designed to solve a much broader set of problems.
Language extensions such as PL/SQL bridge this gap to some extent by adding procedural elements, such as flow-of-control constructs. Another approach is to allow programming language code to be embedded in and interact with the database. For example, Oracle and others include Java in the database, and SQL Server 2005 allows any .NET language to be hosted within the database server process, while PostgreSQL allows functions to be written in a wide variety of languages, including Perl, Tcl, and C.

[edit] Reasons for lack of portability
There are several reasons for this lack of portability between database systems:
The complexity and size of the SQL standard means that most databases do not implement the entire standard.
The standard does not specify database behavior in several important areas (e.g. indexes), leaving it up to implementations of the standard 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.

[edit] SQL keywords
SQL keywords fall into several groups.

[edit] Data retrieval
The most frequently used operation in transactional databases is the data retrieval operation. When restricted to data retrieval commands, SQL acts as a declarative language.
SELECT is used to retrieve zero or more rows from one or more tables in a database. In most applications, SELECT is the most commonly used Data Manipulation Language command. In specifying a SELECT query, the user specifies a description of the desired result set, but they do not specify what physical operations must be executed to produce that result set. Translating the query into an efficient query plan is left to the database system, more specifically to the query optimizer.
Commonly available keywords related to SELECT include:
FROM is used to indicate from which tables the data is to be taken, as well as how the tables JOIN to each other.
WHERE is used to identify which rows to be retrieved, or applied to GROUP BY. WHERE is evaluated before the GROUP BY.
GROUP BY is used to combine rows with related values into elements of a smaller set of rows.
HAVING is used to identify which of the "combined rows" (combined rows are produced when the query has a GROUP BY keyword or when the SELECT part contains aggregates), are to be retrieved. HAVING acts much like a WHERE, but it operates on the results of the GROUP BY and hence can use aggregate functions.
ORDER BY is used to identify which columns are used to sort the resulting data.
Data retrieval is very often combined with data projection; usually it isn't the verbatim data stored in primitive data types that a user is looking for or a query is written to serve. Often the data needs to be expressed differently from how it's stored. SQL allows a wide variety of formulas included in the select list to project data. A common example would be:
SELECT UnitCost * Quantity As TotalCost FROM Orders Example 1:
SELECT * FROM books
WHERE price > 100.00 and price < 150.00
ORDER BY title
This is an example that could be used to get a list of expensive books. It retrieves the records from the books table that have a price field which is greater than 100.00 and less than 150.00. The result is sorted alphabetically by book title. The asterisk (*) means to show all columns of the books table. Alternatively, specific columns could be named. Example 2:
SELECT books.title, count(*) AS Authors
FROM books
JOIN book_authors
ON books.book_number = book_authors.book_number
GROUP BY books.title
Example 2 shows both the use of multiple tables in a join, and aggregation (grouping). This example shows how many authors there are per book. Example output may resemble: Title Authors
---------------------- -------
SQL Examples and Guide 3
The Joy of SQL 1
How to use Wikipedia 2
Pitfalls of SQL 1
How SQL Saved my Dog 1

[edit] Data manipulation
First there are the standard Data Manipulation Language (DML) elements. DML is the subset of the language used to add, update and delete data.
INSERT is used to add zero or more rows (formally tuples) to an existing table.
UPDATE is used to modify the values of a set of existing table rows.
MERGE is used to combine the data of multiple tables. It is something of a combination of the INSERT and UPDATE elements. It is defined in the SQL:2003 standard; prior to that, some databases provided similar functionality via different syntax, sometimes called an "upsert".
DELETE removes zero or more existing rows from a table. Example:
INSERT INTO my_table (field1, field2, field3) VALUES ('test', 'N', NULL);
UPDATE my_table SET field1 = 'updated value' WHERE field2 = 'N';
DELETE FROM my_table WHERE field2 = 'N';

[edit] Transaction Controls
Transaction, if available, can be used to wrap around the DML operations.
BEGIN WORK (or START TRANSACTION, depending on SQL dialect) can be used to mark the start of a database transaction, which either completes completely or not at all.
COMMIT causes all data changes in a transaction to be made permanent.
ROLLBACK causes all data changes since the last COMMIT or ROLLBACK to be discarded, so that the state of the data is "rolled back" to the way it was prior to those changes being requested.
COMMIT and ROLLBACK interact with areas such as transaction control and locking. Strictly, both terminate any open transaction and release any locks held on data. In the absence of a BEGIN WORK or similar statement, the semantics of SQL are implementation-dependent.Example:
BEGIN WORK;
UPDATE inventory SET quantity = quantity - 3 WHERE item = 'pants';
COMMIT;

[edit] Data definition
The second group of keywords is the Data Definition Language (DDL). DDL allows the user to define new tables and associated elements. Most commercial SQL databases have proprietary extensions in their DDL, which allow control over nonstandard features of the database system. The most basic items of DDL are the CREATE,ALTER,RENAME,TRUNCATE and DROP commands.
CREATE causes an object (a table, for example) to be created within the database.
DROP causes an existing object within the database to be deleted, usually irretrievably.
TRUNCATE deletes all data from a table (non-standard, but common SQL command).
ALTER command permits the user to modify an existing object in various ways -- for example, adding a column to an existing table. Example:
CREATE TABLE my_table (
my_field1 INT,
my_field2 VARCHAR (50),
my_field3 DATE NOT NULL,
PRIMARY KEY (my_field1, my_field2)
);

[edit] Data control
The third group of SQL keywords is the Data Control Language (DCL). DCL handles the authorization aspects of data and permits the user to control who has access to see or manipulate data within the database. Its two main keywords are:
GRANT — authorizes one or more users to perform an operation or a set of operations on an object.
REVOKE — removes or restricts the capability of a user to perform an operation or a set of operations. Example:
GRANT SELECT, UPDATE ON my_table TO some_user, another_user.

[edit] Other
ANSI-standard SQL supports -- as a single line comment identifier (some extensions also support curly brackets or C-style /* comments */ for multi-line comments). Example:
SELECT * FROM inventory -- Retrieve everything from inventory table
Some SQL servers allow User Defined Functions

[edit] Criticisms of SQL

The neutrality of this article is disputed.Please see the discussion on the talk page.
Technically, SQL is a declarative computer language for use with "SQL databases". Theorists and some practitioners note that many of the original SQL features were inspired by, but in violation of, the relational model for database management and its tuple calculus realization. Recent extensions to SQL achieved relational completeness, but have worsened the violations, as documented in The Third Manifesto.
In addition, there are also some criticisms about the practical use of SQL:
Implementations are inconsistent and, usually, incompatible between vendors. In particular date and time syntax, string concatenation, nulls, and comparison case sensitivity often vary from vendor-to-vendor.
The language makes it too easy to do a Cartesian join, which results in "run-away" result sets when WHERE clauses are mistyped. Cartesian joins are so rarely used in practice that requiring an explicit CARTESIAN keyword may be warranted.

[edit] 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)
Tutorial D
TQL - Luca Cardelli (May not be relational)
Top's Query Language - A draft language influenced by IBM BS12. Tentatively renamed to SMEQL to avoid confusion with similar projects called TQL.
Hibernate Query Language[2] (HQL) - A Java-based tool that uses modified SQL
Quel introduced in 1974 by the U.C. Berkeley Ingres project.
Object Query Language - Object Data Management Group.
Datalog
LINQ

[edit] See also

[edit] Database systems using SQL
Comparison of relational database management systems
Comparison of truly relational database management systems
Comparison of object-relational database management systems
List of relational database management systems
List of object-relational database management systems
List of hierarchical database management systems

[edit] SQL variants
Comparison of SQL syntax

[edit] References
^ Donald D. Chamberlin and Raymond F. Boyce, 1974. "SEQUEL: A structured English query language", International Conference on Management of Data, Proceedings of the 1974 ACM SIGFIDET (now SIGMOD) workshop on Data description, access and control, Ann Arbor, Michigan, pp. 249–264
Discussion on alleged SQL flaws (C2 wiki)
Web page about FSQL: References and links.
Galindo J., Urrutia A., Piattini M., "Fuzzy Databases: Modeling, Design and Implementation". Idea Group Publishing Hershey, USA, 2005.

[edit] External links
SQL Basics
The 1995 SQL Reunion: People, Projects, and Politics (early history of SQL)
SQL:2003, SQL/XML and the Future of SQL (webcast and podcast with Jim Melton, editor of the SQL standard)
A Gentle Introduction to SQL at SQLzoo
SQL Help and Tutorial
The SQL Language (PostgreSQL specific details included)

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The sbVB DataBase course - A free course on software development using cross-platform C++ and SQL (for any Relational Database, such as Oracle, MSSQL, PostgreSQL, MySQL, DB2, Informix and others)
Topics in database management systems (DBMS) ( view • talk • edit )
ConceptsDatabase Database model Relational database Relational model Relational algebra Primary key - Foreign key - Surrogate key - Superkey - Candidate keyDatabase normalization Referential integrity Relational DBMS Distributed DBMS ACID
ObjectsTrigger View Table Cursor Log Transaction Index Stored procedure Partition
Topics in SQLSelect Insert Update Merge Delete Join Union Create Drop
Implementations of database management systems
Types of implementationsRelational Flat file Deductive Dimensional Hierarchical Object oriented Temporal XML data storesProductsApache Derby Berkeley DB Caché db4o dBASE Firebird Helix DB2 Informix Ingres InterBase Linter Microsoft SQL Server Microsoft Access MySQL OpenLink Virtuoso Oracle PostgreSQL SQLite Sybase IQ Sybase Teradata TimesTen Visual FoxPro Comparison - relational Comparison - object-relational
ComponentsQuery language Query optimizer Query plan ODBC JDBCListsList of object-oriented database management systemsList of relational database management systems