In Informatics, natural language processing, and machine translation, a sublanguage is the language of a restricted domain, particularly a technical domain. In mathematical terms, "a subset of the sentences of a language forms a sublanguage of that language if it is closed under some operations of the language: e.g., if when two members of a subset are operated on, as by and or because, the resultant is also a member of that subset" (Z. S. Harris Language and Information, Columbia U. Press, 1988, p. 34).
The term sublanguage has also sometimes been used to denote a computer language that is a subset of another language. A sublanguage may be restricted syntactically (it accepts a subgrammar of the original language), and/or semantically (the set of possible outcomes for any given program is a subset of the possible outcomes in the original language).
SQL (Structured Query Language) statements are classified in various ways, which can be grouped into sublanguages, commonly: a data query language (DQL), a data definition language (DDL), a data control language (DCL), and a data manipulation language (DML).
In relational database theory, the term "sublanguage", first used for this purpose by E. F. Codd in 1970, refers to a computer language used to define or manipulate the structure and contents of a relational database management system (RDBMS). Typical sublanguages associated with modern RDBMS's are QBE (Query by Example) and SQL (Structured Query Language). In 1985, Codd encapsulated his thinking in twelve rules which every database must satisfy in order to be truly relational. The fifth rule is known as the Comprehensive data sublanguage rule, and states:
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