Last updated
SQL (Structured Query Language)
Paradigm Declarative
Family Query language
Designed by Donald D. Chamberlin
Raymond F. Boyce
Developer ISO/IEC
First appeared1974;47 years ago (1974)
Stable release
SQL:2016 / December 2016;4 years ago (2016-12)
Typing discipline Static, strong
OS Cross-platform
Website www.iso.org/standard/63555.html
Major implementations
Influenced by
SQL (file format)
Filename extension
Internet media type
application/sql [2] [3]
Developed by ISO/IEC
Initial release1986 (1986)
Type of formatDatabase
Standard ISO/IEC 9075
Open format?Yes
Website www.iso.org/standard/63555.html

SQL ( /ˌɛsˌkjuːˈɛl/ ( Loudspeaker.svg listen )S-Q-L, [4] /ˈskwəl/ "sequel"; Structured Query Language) [5] 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). It is particularly useful in handling structured data, i.e. data incorporating relations among entities and variables.


SQL offers two main advantages over older readwrite 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, e.g. with or without an index.

Originally based upon relational algebra and tuple relational calculus, SQL consists of many types of statements, [6] which may be informally classed as sublanguages, commonly: a data query language (DQL), [lower-alpha 1] a data definition language (DDL), [lower-alpha 2] a data control language (DCL), and a data manipulation language (DML). [lower-alpha 3] [7] The scope of SQL includes data query, data manipulation (insert, update and delete), data definition (schema creation and modification), and data access control. Although SQL is essentially a declarative language (4GL), 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". [8] Despite not entirely adhering to the relational model as described by Codd, it became the most widely used database language. [9] [10]

SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987. [11] Since then, the standard has been revised to include a larger set of features. Despite the existence of standards, most SQL code requires at least some changes before being ported to different database systems.


SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd [12] in the early 1970s. [13] This version, initially called SEQUEL (Structured English Query Language), 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. [13]

Chamberlin and Boyce's first attempt at a relational database language was Square, but it was difficult to use due to subscript notation. After moving to the San Jose Research Laboratory in 1973, they began work on SEQUEL. [12] The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley Dynamics Engineering Limited company. [14]

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. [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 introduced the first commercially available implementation of SQL, Oracle V2 (Version2) 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, [12] and most recently, 2016. [16]


A chart showing several of the SQL language elements that compose a single statement

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

Procedural extensions

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:

SourceAbbreviationFull 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 SPLStored Procedural Language
IBM Netezza NZPLSQL [18] (based on Postgres PL/pgSQL)
InvantivePSQL [19] Invantive Procedural SQL (implements SQL/PSM and PL/SQL)
MariaDB SQL/PSM, PL/SQL SQL/Persistent Stored Module (implements SQL/PSM), Procedural Language/SQL (based on Ada) [20]
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 SSPStarkey Stored Procedures
Oracle PL/SQL Procedural Language/SQL (based on Ada)
PostgreSQL PL/pgSQL Procedural Language/PostgreSQL Structured Query Language (based on reduced PL/SQL)
SAP R/3 ABAP Advanced Business Application Programming
Sybase Watcom-SQLSQL Anywhere Watcom-SQL Dialect
Teradata SPLStored Procedural Language

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. Microsoft 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. [21]

Interoperability and standardization


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 [22] and Mimer SQL [23] which strive for standards compliance, though PostgreSQL does not adhere to the standard in how folding of unquoted names is done. The folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, [24] which says that unquoted names should be folded to upper case. [25] Thus, Foo should be equivalent to FOO not foo according to the standard.

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, [26] [27] and lacks a TIME type) [28] and MS SQL Server (before the 2008 version). As a result, SQL code can rarely be ported between database systems without modifications.

Reasons for incompatibility

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

Standardization history

SQL was adopted as a standard by the ANSI in 1986 as SQL-86 [29] and the ISO in 1987. [11] 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 (NIST) data-management standards program certified SQL DBMS compliance with the SQL standard. Vendors now self-certify the compliance of their products. [30]

The original standard declared that the official pronunciation for "SQL" was an initialism: /ˌɛsˌkjuːˈɛl/ ("ess cue el"). [9] Regardless, many English-speaking database professionals (including Donald Chamberlin himself [31] ) use the acronym-like pronunciation of /ˈskwəl/ ("sequel"), [32] mirroring the language's prerelease development name, "SEQUEL". [13] [14] [31]
The SQL standard has gone through a number of revisions:

1986SQL-86SQL-87First formalized by ANSI
1989SQL-89 FIPS 127-1Minor revision that added integrity constraints, adopted as FIPS 127-1
1992 SQL-92 SQL2, FIPS 127-2Major revision (ISO 9075), Entry Level SQL-92 adopted as FIPS 127-2
1999 SQL:1999 SQL3Added regular expression matching, recursive queries (e.g. transitive closure), triggers, support for procedural and control-of-flow statements, nonscalar types (arrays), 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 autogenerated 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. [33]
2008 SQL:2008 Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers, TRUNCATE statement, [34] FETCH clause
2011 SQL:2011 Adds temporal data (PERIOD FOR) [35] (more information at: Temporal database#History). Enhancements for window functions and FETCH clause. [36]
2016 SQL:2016 Adds row pattern matching, polymorphic table functions, JSON
2019SQL:2019Adds Part 15, multidimensional arrays (MDarray type and operators)

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.

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 such as video, audio, and spatial data. Interested parties may purchase SQL standards documents from ISO, [37] IEC or ANSI. A draft of SQL:2008 is freely available as a zip archive. [38]

Anatomy of SQL Standard

The SQL standard is divided into 10 parts, but with gaps in the numbering due to the withdrawal of outdated parts.

  • ISO/IEC 9075-1:2016 Part 1: Framework (SQL/Framework). It provides logical concepts. [39]
  • ISO/IEC 9075-2:2016 Part 2: Foundation (SQL/Foundation). It contains the most central elements of the language and consists of both mandatory and optional features.
  • ISO/IEC 9075-3:2016 Part 3: Call-Level Interface (SQL/CLI). It defines interfacing components (structures, procedures, variable bindings) that can be used to execute SQL statements from applications written in Ada, C respectively C++, COBOL, Fortran, MUMPS, Pascal or PL/I. (For Java see part 10.) SQL/CLI is defined in such a way that SQL statements and SQL/CLI procedure calls are treated as separate from the calling application's source code. Open Database Connectivity is a well-known superset of SQL/CLI. This part of the standard consists solely of mandatory features.
  • ISO/IEC 9075-4:2016 Part 4: Persistent stored modules (SQL/PSM). It standardizes procedural extensions for SQL, including flow of control, condition handling, statement condition signals and resignals, cursors and local variables, and assignment of expressions to variables and parameters. In addition, SQL/PSM formalizes declaration and maintenance of persistent database language routines (e.g., "stored procedures"). This part of the standard consists solely of optional features.
  • ISO/IEC 9075-9:2016 Part 9: Management of External Data (SQL/MED). It provides extensions to SQL that define foreign-data wrappers and datalink types to allow SQL to manage external data. External data is data that is accessible to, but not managed by, an SQL-based DBMS. This part of the standard consists solely of optional features.
  • ISO/IEC 9075-10:2016 Part 10: Object language bindings (SQL/OLB). It defines the syntax and semantics of SQLJ, which is SQL embedded in Java (see also part 3). The standard also describes mechanisms to ensure binary portability of SQLJ applications, and specifies various Java packages and their contained classes. This part of the standard consists solely of optional features. Unlike SQL/OLB JDBC defines an API and is not part of the SQL standard.[ citation needed ]
  • ISO/IEC 9075-11:2016 Part 11: Information and definition schemas (SQL/Schemata). It defines the Information Schema and Definition Schema, providing a common set of tools to make SQL databases and objects self-describing. These tools include the SQL object identifier, structure and integrity constraints, security and authorization specifications, features and packages of ISO/IEC 9075, support of features provided by SQL-based DBMS implementations, SQL-based DBMS implementation information and sizing items, and the values supported by the DBMS implementations. [40] This part of the standard contains both mandatory and optional features.
  • ISO/IEC 9075-13:2016 Part 13: SQL Routines and types using the Java TM programming language (SQL/JRT). It specifies the ability to invoke static Java methods as routines from within SQL applications ('Java-in-the-database'). It also calls for the ability to use Java classes as SQL structured user-defined types. This part of the standard consists solely of optional features.
  • ISO/IEC 9075-14:2016 Part 14: XML-Related Specifications (SQL/XML). It specifies SQL-based extensions for using XML in conjunction with SQL. The XML data type is introduced, as well as several routines, functions, and XML-to-SQL data type mappings to support manipulation and storage of XML in an SQL database. [33] This part of the standard consists solely of optional features.[ citation needed ]
  • ISO/IEC 9075-15:2019 Part 15: Multi-dimensional arrays (SQL/MDA). It specifies a multidimensional array type (MDarray) for SQL, along with operations on MDarrays, MDarray slices, MDarray cells, and related features. This part of the standard consists solely of optional features.

Extensions to the ISO/IEC Standard

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.

  • ISO/IEC 13249-1:2016 Part 1: Framework
  • ISO/IEC 13249-2:2003 Part 2: Full-Text
  • ISO/IEC 13249-3:2016 Part 3: Spatial
  • ISO/IEC 13249-5:2003 Part 5: Still image
  • ISO/IEC 13249-6:2006 Part 6: Data mining
  • ISO/IEC 13249-7:2013 Part 7: History
  • ISO/IEC 13249-8:xxxx Part 8: Metadata Registry Access MRA (work in progress)

Technical reports

ISO/IEC 9075 is also accompanied by a series of Technical Reports, published as ISO/IEC TR 19075. These Technical Reports explain the justification for and usage of some features of SQL, giving examples where appropriate. The Technical Reports are non-normative; if there is any discrepancy from 9075, the text in 9075 holds. Currently available 19075 Technical Reports are:

  • ISO/IEC TR 19075-1:2011 Part 1: XQuery Regular Expression Support in SQL
  • ISO/IEC TR 19075-2:2015 Part 2: SQL Support for Time-Related Information
  • ISO/IEC TR 19075-3:2015 Part 3: SQL Embedded in Programs using the Java programming language
  • ISO/IEC TR 19075-4:2015 Part 4: SQL with Routines and types using the Java programming language
  • ISO/IEC TR 19075-5:2016 Part 5: Row Pattern Recognition in SQL
  • ISO/IEC TR 19075-6:2017 Part 6: SQL support for JavaScript Object Notation (JSON)
  • ISO/IEC TR 19075-7:2017 Part 7: Polymorphic table functions in SQL
  • ISO/IEC TR 19075-8:2019 Part 8: Multi-Dimensional Arrays (SQL/MDA)
  • ISO/IEC TR 19075-9:2020 Part 9: Online analytic processing (OLAP) capabilities


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

Distributed Relational Database Architecture (DRDA) was designed by a work group within IBM from 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. Distributed SQL processing ala DRDA is distinctive from contemporary distributed SQL databases.



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 returns strictly to the original foundation: for example, see The Third Manifesto .

Other criticisms

Chamberlin discusses four historical criticisms of SQL in a 2012 paper: [12]

Orthogonality and completeness

Early specifications did not support major features, such as primary keys. Result sets could not be named, and subqueries had not been defined. These were added in 1992. [12]


The concept of Null is the subject of some debates. The Null marker indicates the absence of a value, and is distinct from a value of 0 for an integer column or an empty string for a text column. The concept of Nulls enforces the 3-valued-logic in SQL, which is a concrete implementation of the general 3-valued logic.


Another popular criticism is that it allows duplicate rows, making integration with languages such as Python, whose data types might make accurately representing the data difficult, [12] in terms of parsing and by the absence of modularity. [44]

This can be avoided declaring a unique constraint with one or more fields that identifies uniquely a row in the table. That constraint could also become the primary key of the table.

Impedance mismatch

In a similar sense to object–relational impedance mismatch, a mismatch occurs between the declarative SQL language and the procedural languages in which SQL is typically embedded.

SQL data types

The SQL standard defines three kinds of data types:[ citation needed ]

  • predefined data types
  • constructed types
  • user-defined types.

Constructed types are one of ARRAY, MULTISET, REF(erence), or ROW. User-defined types are comparable to classes in object-oriented language with their own constructors, observers, mutators, methods, inheritance, overloading, overwriting, interfaces, and so on. Predefined data types are intrinsically supported by the implementation.

Predefined data types

  • Character types
  • Character (CHAR)
  • Character varying (VARCHAR)
  • Character large object (CLOB)
  • National character types
  • National character (NCHAR)
  • National character varying (NCHAR VARYING)
  • National character large object (NCLOB)
  • Binary types
  • Binary (BINARY)
  • Binary varying (VARBINARY)
  • Binary large object (BLOB)
  • Numeric types
  • Approximate numeric types (FLOAT, REAL, DOUBLE PRECISION)
  • Decimal floating-point type (DECFLOAT)
  • Datetime types (DATE, TIME, TIMESTAMP)
  • Interval type (INTERVAL)
  • Boolean
  • XML
  • JSON

See also


  1. Formally, "SQL-data" statements excluding "SQL-data change" statements; this is primarily the Select statement.
  2. Formally, "SQL-schema" statements.
  3. Formally, "SQL-data change" statements

Related Research Articles

Object–relational database Database management system

An object–relational database (ORD), or object–relational database management system (ORDBMS), is a database management system (DBMS) similar to a relational database, but with an object-oriented database model: objects, classes and inheritance are directly supported in database schemas and in the query language. In addition, just as with pure relational systems, it supports extension of the data model with custom data types and methods.

IBM Db2 Family Relational model database server

Db2 is a family of data management products, including database servers, developed by IBM. They initially supported the relational model, but were extended to support object–relational features and non-relational structures like JSON and XML. The brand name was originally styled as DB/2, then DB2 until 2017 and finally changed to its present form.

In the context of SQL, data definition or data description language (DDL) is a syntax for creating and modifying database objects such as tables, indices, and users. DDL statements are similar to a computer programming language for defining data structures, especially database schemas. Common examples of DDL statements include CREATE, ALTER, and DROP.

A data control language (DCL) is a syntax similar to a computer programming language used to control access to data stored in a database (Authorization). In particular, it is a component of Structured Query Language (SQL). Data Control Language is one of the logical group in SQL Commands. SQL is the standard language for relational database management systems. SQL statements are used to perform tasks such as insert data to a database, delete or update data in a database, or retrieve data from a database.

An XML database is a data persistence software system that allows data to be specified, and sometimes stored, in XML format. This data can be queried, transformed, exported and returned to a calling system. XML databases are a flavor of document-oriented databases which are in turn a category of NoSQL database.

The following tables compare general and technical information for a number of relational database management systems. Please see the individual products' articles for further information. Unless otherwise specified in footnotes, comparisons are based on the stable versions without any add-ons, extensions or external programs.

Null (SQL) Marker used in SQL databases to indicate a value does not exist

Null or NULL is a special marker used in Structured Query Language to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfil the requirement that all true relational database management systems (RDMS) support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent Null in database theory. In SQL, NULL is a reserved word used to identify this marker.

Simple Features is a set of standards that specify a common storage and access model of geographic feature made of mostly two-dimensional geometries used by geographic information systems. It is formalized by both the Open Geospatial Consortium (OGC) and the International Organization for Standardization (ISO).

SQLJ is a working title for efforts to combine Java and SQL. It was a common effort started around 1997 by engineers from IBM, Oracle, Compaq, Informix, Sybase, Cloudscape and Sun Microsystems.

SQL/PSM is an ISO standard mainly defining an extension of SQL with a procedural language for use in stored procedures. Initially published in 1996 as an extension of SQL-92, SQL/PSM was later incorporated into the multi-part SQL:1999 standard, and has been part 4 of that standard since then, most recently in SQL:2016. The SQL:1999 part 4 covered less than the original PSM-96 because the SQL statements for defining, managing, and invoking routines were actually incorporated into part 2 SQL/Foundation, leaving only the procedural language itself as SQL/PSM. The SQL/PSM facilities are still optional as far as the SQL standard is concerned; most of them are grouped in Features P001-P008.

The SQL/MED extension to the SQL standard is defined by ISO/IEC 9075-9:2008. SQL/MED provides extensions to SQL that define foreign-data wrappers and datalink types to allow SQL to manage external data. External data is data that is accessible to, but not managed by, an SQL-based DBMS. This standard can be used in the development of federated database systems.

SQL/JRT, or SQL Routines and Types for the Java Programming Language, is an extension to the SQL standard first published as ISO/IEC 9075-13:2002. SQL/JRT specifies the ability to invoke static Java methods as routines from within SQL applications, commonly referred to as "Java stored procedures". SQL/JRT also calls for the ability to use Java classes as SQL structured user-defined types. The two parts of the extension originate from the earlier ANSI SQLJ part 1 and 2 standards

SQL/XML or XML-Related Specifications is part 14 of the Structured Query Language (SQL) specification. In addition to the traditional predefined SQL data types like NUMERIC, CHAR, TIMESTAMP, ... it introduces the predefined data type XML together with constructors, several routines, functions, and XML-to-SQL data type mappings to support manipulation and storage of XML in a SQL database.

SQL:1999 was the fourth revision of the SQL database query language. It introduced many new features, many of which required clarifications in the subsequent SQL:2003. In the meanwhile SQL:1999 is deprecated.

A hierarchical query is a type of SQL query that handles hierarchical model data. They are special cases of more general recursive fixpoint queries, which compute transitive closures.

SQL:2006 or ISO/IEC 9075:2006 standard is a revision part 14 of the ISO standard for the SQL database query language. It is not a revision of the complete SQL standard.

SQL:2011 or ISO/IEC 9075:2011 is the seventh revision of the ISO (1987) and ANSI (1986) standard for the SQL database query language. It was formally adopted in December 2011. The standard consists of 9 parts which are described in detail in SQL. The next version is SQL:2016.

SQL:2016 or ISO/IEC 9075:2016 is the eighth revision of the ISO (1987) and ANSI (1986) standard for the SQL database query language. It was formally adopted in December 2016. The standard consists of 9 parts which are described in some detail in SQL.

The syntax of the SQL programming language is defined and maintained by ISO/IEC SC 32 as part of ISO/IEC 9075. This standard is not freely available. Despite the existence of the standard, SQL code is not completely portable among different database systems without adjustments.

This list includes SQL reserved words as specified in the SQL:2016 standard, and also some products' reserved words.


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SQL standards documents

ITTF publicly available standards and technical reports

The ISO/IEC Information Technology Task Force publishes publicly available standards including SQL. Technical Corrigenda (corrections) and Technical Reports (discussion documents) are published there.

SQL -- Part 1: Framework (SQL/Framework)

Draft documents

Formal SQL standards are available from ISO and ANSI for a fee. For informative use, as opposed to strict standards compliance, late drafts often suffice.