SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system, or for stream processing in a relational data stream management system. It is particularly useful in handling structured data, i.e. data incorporating relations among entities and variables.
SQL offers three 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. Finally, SQL uses a human-readable syntax that allows users to be quickly productive without a requirement for long-term, technical training.
Originally based upon relational algebra and tuple relational calculus, SQL consists of many types of statements, which may be informally classed as sublanguages, commonly: a data query language, a data definition language, a data control language, and a 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 utilize 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, it 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 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.
HistorySQL 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 quasi-relational 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 of 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-based Hawker 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. saw the potential of the concepts described by Codd, Chamberlin, and Boyce, and developed their own SQL-based RDMS with aspirations of selling it to the U.S. Navy, Central Intelligence Agency, and other U.S. government agencies. In June 1979, Relational Software, Inc. introduced the first commercially available implementation 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 and, most recently, 2016.
DesignSQL 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.
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.
SyntaxThe 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.
|ANSI/ISO Standard||SQL/PSM||SQL/Persistent Stored Modules|
|Interbase / Firebird||PSQL||Procedural SQL|
|IBM DB2||SQL PL||SQL Procedural Language|
|IBM Informix||SPL||Stored Procedural Language|
|Invantive||PSQL||Invantive Procedural SQL|
|MariaDB||SQL/PSM, PL/SQL||SQL/Persistent Stored Module, Procedural Language/SQL|
|Microsoft / Sybase||T-SQL||Transact-SQL|
|Mimer SQL||SQL/PSM||SQL/Persistent Stored Module|
|MySQL||SQL/PSM||SQL/Persistent Stored Module|
|MonetDB||SQL/PSM||SQL/Persistent Stored Module|
|NuoDB||SSP||Starkey Stored Procedures|
|PostgreSQL||PL/pgSQL||Procedural Language/PostgreSQL Structured Query Language|
|SAP R/3||ABAP||Advanced Business Application Programming|
|Sybase||Watcom-SQL||SQL Anywhere Watcom-SQL Dialect|
|Teradata||SPL||Stored Procedural Language|
Interoperability and standardizationSQL 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,
Fooshould be equivalent to
fooaccording to the standard.
Popular implementations of SQL commonly omit support for basic features of Standard SQL, such as the
TIMEdata 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.
There are several reasons for this lack of portability between database systems:
- The complexity and size of the SQL standard means that most implementors do not support the entire standard.
- The standard does not specify database behavior in several important areas, 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.
- There is little commercial incentive for vendors to make it easier for users to change database suppliers.
- Users evaluating database software tend to place other factors such as performance higher in their priorities than standards conformance.
ISO/IEC 9075 is complemented by ISO/IEC 13249: SQL Multimedia and Application Packages, which defines SQL based interfaces and packages to widely spread applications like video, audio and spatial data.
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 pre-release development name, "SEQUEL". The SQL standard has gone through a number of revisions:
|1986||SQL-86||SQL-87||First formalized by ANSI.|
|1989||SQL-89||FIPS 127-1||Minor revision that added integrity constraints, adopted as FIPS 127-1.|
|1992||SQL-92||SQL2, FIPS 127-2||Major revision, Entry Level SQL-92 adopted as FIPS 127-2.|
|1999||SQL3||Added regular expression matching, recursive queries, triggers, support for procedural and control-of-flow statements, non-scalar types, and some object-oriented features. Support for embedding SQL in Java and vice versa.|
|2003||Introduced XML-related features, window functions, standardized sequences, and columns with auto-generated values.|
|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, to concurrently access ordinary SQL-data and XML documents.|
|2008||Legalizes ORDER BY outside cursor definitions. Adds INSTEAD OF triggers, TRUNCATE statement, FETCH clause.|
|2011||Adds temporal data . Enhancements for window functions and FETCH clause.|
|2016||Adds row pattern matching, polymorphic table functions, JSON.|
|2019||SQL:2019||Adds Part 15, multidimensional arrays.|
Interested parties may purchase SQL standards documents from ISO, IEC or ANSI. A draft of SQL:2008 is freely available as a zip archive.
The SQL standard is divided into ten parts. There are gaps in the numbering due to the withdrawal of outdated parts.
- ISO/IEC 9075-1:2016 Part 1: Framework. It provides logical concepts.
- ISO/IEC 9075-2:2016 Part 2: 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. It defines interfacing components that can be used to execute SQL statements from applications written in Ada, C respectively C++, COBOL, Fortran, MUMPS, Pascal or PL/I. 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. 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. This part of the standard consists solely of optional features.
- ISO/IEC 9075-9:2016 Part 9: Management of External Data. 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. It defines the syntax and semantics of SQLJ, which is SQL embedded in Java. 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. 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. It specifies the ability to invoke static Java methods as routines from within SQL applications. 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. 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. It specifies a multidimensional array type for SQL, along with operations on MDarrays, MDarray slices, MDarray cells, and related features. This part of the standard consists solely of optional features.
- 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
- 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 JavaTM programming language
- ISO/IEC TR 19075-4:2015 Part 4: SQL with Routines and types using the JavaTM 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
- .QL: object-oriented Datalog
- 4D Query Language
- 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.
- HTSQL: URL based query method
- IBM Business System 12 : one of the first fully relational database management systems, introduced in 1982
- jOOQ: SQL implemented in Java as an internal domain-specific language
- Java Persistence Query Language : The query language used by the Java Persistence API and Hibernate persistence library
- LINQ: Runs SQL statements written like language constructs to query collections directly from inside.Net code.
- Object Query Language
- QBE created by Moshè Zloof, IBM 1977
- Quel introduced in 1974 by the U.C. Berkeley Ingres project.
- Tutorial D
Distributed SQL processing
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.
CriticismsChamberlin's 2012 paper discusses four historical criticisms of SQL:
Orthogonality and completenessEarly specifications did not support major features, such as primary keys. Result sets could not be named, and sub-queries had not been defined. These were added in 1992.debates. The Null marker indicates that there is no value, even no 0 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.
DuplicatesAnother popular criticism is that it allows duplicate rows, making integration with languages such as Python, whose data types might make it difficult to accurately represent 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.
Impedance mismatchIn a similar sense to Object-relational impedance mismatch, there is a mismatch between the declarative SQL language and the procedural languages that SQL is typically embedded in.
Data Integrity CategoriesMain data integrity categories of each RDBMS.
Entity integrityEstablishes that within the table the primary key has a unique value for each row, checking the uniqueness of the value of the primary key avoiding that there are duplicated rows in a table.
Domain integrityRestricts the type, format, and value range that applies to valid entries for a column within a table
Referential integrityMakes rows in a table that are being used by other records impossible to delete
User-defined integrityOther specific rules not included above apply
SQL data typesThe SQL standard defines three kinds of data types:
Predefined data types are:
- Character Types
- Binary Types
- Numeric Types
- Datetime Types
- Interval Type