sql

SQL keywords

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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.

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.

Transaction controls

Transactions, 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.

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.

Standardization

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SQL was adopted as a standard by ANSI (American National Standards Institute) in 1986 and ISO (International Organization for Standardization) in 1987. However, since the dissolution of the NIST data management standards program in 1996 there has been no certification for compliance with the SQL standard so vendors must be relied on to self-certify.

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).
2006	SQL:2006		ISO/IEC 9075-14:2006 defines ways in which SQL can be used in conjunction with XML. It defines ways of importing and storing XML data in an SQL database, manipulating it within the database and publishing both XML and conventional SQL-data in XML form. In addition, it provides facilities that permit applications to integrate into their SQL code the use of XQuery, the XML Query Language published by the World Wide Web Consortium (W3C), to concurrently access ordinary SQL-data and XML documents.

The SQL standard is not freely available. SQL:2003 and SQL:2006 may be purchased from ISO or ANSI. A late draft of SQL:2003 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.

History

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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 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. 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).

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 V2 beat IBM's release of the System/38 to the market by a few weeks.

about

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Structured Query Language (SQL) is the most popular computer language used to create, retrieve, update and delete data from relational database management systems. SQL has been standardized by both ANSI and ISO.

SQL is commonly spoken either as the names of the letters ess-cue-el (IPA: [ˈɛsˈkjuˈɛl]), or like the word sequel (IPA: [ˈsiːkwəl]). The official pronunciation of SQL according to ANSI is ess-cue-el. However, each of the major database products (or projects) containing the letters SQL has its own convention: MySQL is officially and commonly pronounced "My Ess Cue El"; PostgreSQL is expediently pronounced postgres (being the name of the predecessor to PostgreSQL); and Microsoft SQL Server is commonly spoken as Microsoft-sequel-server.