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SQL

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SQL (commonly expanded to Structured Query Language — see History for the term's derivation) is the most popular computer language used to create, modify, retrieve and manipulate data from relational database management systems. The language has evolved beyond its original purpose to support object-relational database management systems. It is an ANSI/ISO standard.


Scope

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

Although SQL is defined by both ANSI and ISO, there are many extensions to and variations on the version of the language defined by these standards bodies. Many of these extensions are of a proprietary nature, such as Oracle Corporation's PL/SQL or Sybase, IBM's SQL PL (SQL Procedural Language) and Microsoft's Transact-SQL. It is also not uncommon for commercial implementations to omit support for basic features of the standard, such as the DATE or TIME data types, preferring some variant of their own. As a result, in contrast to ANSI C or ANSI Fortran, which can usually be ported from platform to platform without major structural changes, SQL code can rarely be ported between database systems without major modifications. There are several reasons for this lack of portability between database systems:

  • the complexity and size of the SQL standard means that most databases do not implement the entire standard.
  • the standard does not specify database behavior in several important areas (e.g. indexes), leaving it up to implementations of the standard to decide how to behave.
  • the SQL standard precisely specifies the syntax that a conformant database system must implement. However, the standard's specification of the semantics of language constructs is less well-defined, leading to areas of ambiguity.
  • many database vendors have large existing customer bases; where the SQL standard conflicts with the prior behavior of the vendor's database, the vendor may be unwilling to break backward compatibility.
  • some believe the lack of compatibility between database systems is intentional in order to ensure vendor lock-in.

SQL is designed for a specific, limited purpose — querying data contained in a relational database. As such, it is a set-based, declarative computer language rather than an imperative language such as C or BASIC which, being programming languages, are designed to solve a much broader set of problems. Language extensions such as PL/SQL are designed to address this by turning SQL into a full-fledged programming language while maintaining the advantages of SQL. Another approach is to allow programming language code to be embedded in and interact with the database. For example, Oracle and others include Java in the database, while PostgreSQL allows functions to be written in a wide variety of languages, including Perl, Tcl, and C.

One joke about SQL is that "SQL is neither structured, nor is it limited to queries, nor is it a language." This is founded on the notion that pure SQL is not a classic programming language since it is not Turing-complete. On the other hand, however, it is a programming language because it has a grammar, syntax, and programmatic purpose and intent. The joke recalls Voltaire's remark that the Holy Roman Empire was "neither holy, nor Roman, nor an empire."

SQL contrasts with the more powerful database-oriented fourth-generation programming languages such as Focus or SAS in its relative functional simplicity and simpler command set. This greatly reduces the degree of difficulty involved in maintaining SQL source code, but it also makes programming such questions as 'Who had the top ten scores?' more difficult, leading to the development of procedural extensions, discussed above. However, it also makes it possible for SQL source code to be produced (and optimized) by software, leading to the development of a number of natural language database query languages, as well as 'drag and drop' database programming packages with 'object oriented' interfaces. Often these allow the resultant SQL source code to be examined, for educational purposes, further enhancement, or to be used in a different environment.


Database systems using SQL

Criticisms of SQL

Technically, SQL is a declarative computer language for use with "SQL databases". Theorists and some practitioners note that many of the original SQL features were inspired by, but in violation of, the relational model for database management and its tuple calculus realisation. Recent extensions to SQL achieved relational completeness, but have worsened the violations, as documented in The Third Manifesto.

In addition, there are also some criticisms about the practical use of SQL:

  • The language syntax is rather complex (sometimes called "COBOL-like").
  • It does not provide a standard way, or at least a commonly-supported way, to split large commands into multiple smaller ones that reference each other by name. This tends to result in "run-on SQL sentences" and may force one into a deep hierarchical nesting when a graph-like (reference-by-name) approach may be more appropriate and better repetition-factoring.
  • Implementations are inconsistent and, usually, incompatible between vendors.
  • For larger statements, it is often difficult to factor repeated patterns and expressions into one or fewer places to avoid repetition and avoid having to make the same change to different places in a given statement.
  • Confusion about the difference between value-to-column assignment in UPDATE and INSERT syntax.

Alternatives to SQL

A distinction should be made between alternatives to relational and alternatives to SQL. The list below are proposed alternatives to SQL, but are still (nominally) relational. See navigational database for alternatives to relational.


See also

Wikibook

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Books

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