SQL

Structured Query Language is a domain-specific language used to manage data, especially in a relational database management system. It is particularly useful in handling structured data, i.e., data incorporating relations among entities and variables.
Introduced in the 1970s, SQL offered two main advantages over older read-write APIs such as ISAM or VSAM. Firstly, it introduced the concept of accessing many records with one single command. Secondly, it eliminates the need to specify how to reach a record, i.e., with or without an index.
Originally based upon relational algebra and tuple relational calculus, SQL consists of many types of statements, which may be informally classed as sublanguages, commonly: data query language, data definition language, data control language, and data manipulation language.
The scope of SQL includes data query, data manipulation, data definition, and data access control. Although SQL is essentially a declarative language, it also includes procedural elements.
SQL was one of the first commercial languages to use Edgar F. Codd's relational model. The model was 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, SQL became the most widely used database language.
SQL became a standard of the American National Standards Institute in 1986 and of the International Organization for Standardization in 1987. Since then, the standard has been revised multiple times to include a larger set of features and incorporate common extensions. Despite the existence of standards, virtually no implementations in existence adhere to it fully, and most SQL code requires at least some changes before being ported to different database systems.

History

SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd in the early 1970s. This version, initially called SEQUEL, was designed to manipulate and retrieve data stored in IBM's original quasirelational database management system, System R, which a group at IBM San Jose Research Laboratory had developed during the 1970s.
Chamberlin and Boyce's first attempt at a relational database language was SQUARE, but it was difficult to use due to subscript/superscript notation. After moving to the San Jose Research Laboratory in 1973, they began work on a sequel to SQUARE. The original name SEQUEL, which is widely regarded as a pun on QUEL, the query language of Ingres, was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley Dynamics Engineering Limited company. The label SQL later became the acronym for Structured Query Language.
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 IBM Db2, which were commercially available in 1979, 1981, and 1983, respectively. IBM's endorsement caused the industry to move to SQL from alternatives like QUEL.
In the late 1970s, Relational Software, Inc. 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 introduced one of the first commercially available implementations of SQL, Oracle V2 for VAX computers.
By 1986, ANSI and ISO standard groups officially adopted the standard "Database Language SQL" language definition. New versions of the standard were published in 1989, 1992, 1996, 1999, 2003, 2006, 2008, 2011, 2016 and, most recently, 2023.

Interoperability and standardization

Overview

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. PostgreSQL and Mimer SQL strive for standards compliance, though PostgreSQL does not adhere to the standard in all cases. For example, the folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, which says that unquoted names should be folded to upper case. Thus, according to the standard, Foo should be equivalent to FOO, not foo.
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 and MS SQL Server. As a result, SQL code can rarely be ported between database systems without modifications.

Reasons for incompatibility

Several reasons for the lack of portability between database systems include:

The complexity and size of the SQL standard means that most implementers do not support the entire standard.
The SQL standard does not specify the database behavior in some important areas, leaving implementations to decide how to behave.
The SQL standard defers some decisions to individual implementations, such as how to name a results column that was not named explicitly.
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 ambiguity.
Many database vendors have large existing customer bases; where the newer version of the SQL standard conflicts with the prior behavior of the vendor's database, the vendor may be unwilling to break backward compatibility.
Little commercial incentive exists for vendors to make changing database suppliers easier.
Users evaluating database software tend to place other factors such as performance higher in their priorities than standards conformance.
Standardization history

SQL was adopted as a standard by the ANSI in 1986 as SQL-86 and the ISO in 1987. It is maintained by ISO/IEC JTC 1, Information technology, Subcommittee SC 32, Data management and interchange.
Until 1996, the National Institute of Standards and Technology 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: . Regardless, many English-speaking database professionals use the acronym-like pronunciation of , mirroring the language's prerelease development name, "SEQUEL".
The SQL standard has gone through a number of revisions:

Year	Official standard	Informal name	Comments
1986 1987	ANSI X3.135:1986 ISO/IEC 9075:1987 FIPS PUB 127	SQL-86 SQL-87	First formalized by ANSI, adopted as FIPS PUB 127
1989	ANSI X3.135-1989 ISO/IEC 9075:1989 FIPS PUB 127-1	SQL-89	Minor revision that added integrity constraints, adopted as FIPS PUB 127-1
1992	ANSI X3.135-1992 ISO/IEC 9075:1992 FIPS PUB 127-2	SQL-92 SQL2	Major revision, Entry Level SQL-92, adopted as FIPS PUB 127-2
1999	ISO/IEC 9075:1999	SQL:1999 SQL3	Added regular expression matching, recursive queries, triggers, support for procedural and control-of-flow statements, nonscalar types, and some object-oriented features, support for embedding SQL in Java and vice versa
2003	ISO/IEC 9075:2003	SQL:2003	Introduced XML-related features, window functions, standardized sequences, and columns with autogenerated values
2006		SQL:2006	Adds Part 14, 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, to concurrently access ordinary SQL-data and XML documents.
2008	ISO/IEC 9075:2008	SQL:2008	Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers, TRUNCATE statement, FETCH clause
2011	ISO/IEC 9075:2011	SQL:2011	Adds temporal data . Enhancements for window functions and FETCH clause.
2016	ISO/IEC 9075:2016	SQL:2016	Adds row pattern matching, polymorphic table functions, operations on JSON data stored in character string fields
2019	ISO/IEC 9075-15:2019	SQL:2019	Adds Part 15, multidimensional arrays
2023	ISO/IEC 9075:2023	SQL:2023	Adds data type JSON ; Adds Part 16, Property Graph Queries

Current standard

The standard is commonly denoted by the pattern: ISO/IEC 9075-n:yyyy Part n: title, or, as a shortcut, ISO/IEC 9075. Interested parties may purchase the standards documents from ISO, IEC, or ANSI. Some old drafts are freely available.
ISO/IEC 9075 is complemented by ISO/IEC 13249: SQL Multimedia and Application Packages and some Technical reports.

Syntax

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

Clauses, which are constituent components of statements and queries.
Expressions, which can produce either scalar values, or tables consisting of columns and rows of data
Predicates, which specify conditions that can be evaluated to SQL three-valued logic or Boolean truth values and are used to limit the effects of statements and queries, or to change program flow.
Queries, which retrieve the data based on specific criteria. This is an important element of SQL.
Statements, which may have a persistent effect on schemata and data, or may control transactions, program flow, connections, sessions, or diagnostics.
* SQL statements also include the semicolon statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.
Insignificant whitespace is generally ignored in SQL statements and queries, making it easier to format SQL code for readability.