SQL offers 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, e.g. with or without an index.
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. The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-basedHawker Siddeley Dynamics Engineering Limited company.
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.
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, and most recently, 2016.
A chart showing several of the SQL language elements that compose a single statement
The SQL language is subdivided into several language elements, including:
Clauses, which are constituent components of statements and queries. (In some cases, these are optional.)
Predicates, which specify conditions that can be evaluated to SQL three-valued logic (3VL) (true/false/unknown) or Booleantruth 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.
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 and Mimer SQL 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, which says that unquoted names should be folded to upper case. 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, 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.
Reasons for incompatibility
Several reasons for this 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 standard does not specify database behavior in several important areas (e.g. indices, file storage...), leaving implementations 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 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 (see vendor lock-in).
Users evaluating database software tend to place other factors such as performance higher in their priorities than standards conformance.
The original standard declared that the official pronunciation for "SQL" was an initialism: /ˌɛsˌkjuːˈɛl/ ("ess cue el"). Regardless, many English-speaking database professionals (including Donald Chamberlin himself) use the acronym-like pronunciation of /ˈsiːkwəl/ ("sequel"), mirroring the language's prerelease development name, "SEQUEL". The SQL standard has gone through a number of revisions:
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.
Adds Part 15, multidimensional arrays (MDarray type and operators)
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, IEC or ANSI. A draft of SQL:2008 is freely available as a zip archive.
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.
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.
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. 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. This part of the standard consists solely of optional features.
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)
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 19075-7:2017 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.
Datalog: critics suggest that Datalog has two advantages over SQL: it has cleaner semantics, which facilitates program understanding and maintenance, and it is more expressive, in particular for recursive queries.
Quel introduced in 1974 by the U.C. Berkeley Ingres project, including the still-available POSTQUEL supported by Postgres, based on the Relational Calculus, which Stonebraker and others maintained was a superior approach to that of the Relational Algebra on which SQL is based
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.
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. However, no known proof exists that such uniqueness cannot be added to SQL itself, or at least a variation of SQL. In other words, it's quite possible that SQL can be "fixed" or at least improved in this regard such that the industry may not have to switch to a completely different query language to obtain uniqueness. Debate on this remains open.
Chamberlin discusses four historical criticisms of SQL in a 2012 paper:
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.
The concept of Null is the subject of some debates. The Null marker indicates that no value, even no 0 exists for an integer column or a string of length 0 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, in terms of parsing and by the absence of modularity.
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.
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.
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^SQL-92, 4.22 SQL-statements, 4.22.1 Classes of SQL-statements "There are at least five ways of classifying SQL-statements:", 4.22.2, SQL statements classified by function "The following are the main classes of SQL-statements:"; SQL:2003 4.11 SQL-statements, and later revisions.
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Discussion on alleged SQL flaws (C2 wiki)
C. J. Date with Hugh Darwen: A Guide to the SQL standard : a users guide to the standard database language SQL, 4th ed., Addison Wesley, USA 1997, ISBN 978-0-201-96426-4
SQL standards documents
ITTF publicly available standards and technical reports
The ISO/IECInformation 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)
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.
SQLat Wikipedia's sister projects
Definitions from Wiktionary
Media from Wikimedia Commons
Textbooks from Wikibooks
Resources from Wikiversity
1995 SQL Reunion: People, Projects, and Politics, by Paul McJones (ed.): transcript of a reunion meeting devoted to the personal history of relational databases and SQL.
American National Standards Institute. X3H2 Records, 1978–1995 Charles Babbage Institute Collection documents the H2 committee's development of the NDL and SQL standards.
Oral history interview with Donald D. Chamberlin Charles Babbage Institute In this oral history Chamberlin recounts his early life, his education at Harvey Mudd College and Stanford University, and his work on relational database technology. Chamberlin was a member of the System R research team and, with Raymond F. Boyce, developed the SQL database language. Chamberlin also briefly discusses his more recent research on XML query languages.
Comparison of Different SQL Implementations This comparison of various SQL implementations is intended to serve as a guide to those interested in porting SQL code between various RDBMS products, and includes comparisons between SQL:2008, PostgreSQL, DB2, MS SQL Server, MySQL, Oracle, and Informix.
Event stream processing with SQL - An introduction to real-time processing of streaming data with continuous SQL queries
BNF Grammar for ISO/IEC 9075:2003, part 2 SQL/Framework