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A query includes a list of columns to include in the final result, normally immediately following the query returns a list of expensive books.

SQL allows the use of expressions in the select list to project data, as in the following example, which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.

Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. While joins and other table operations provide computationally superior (i.e.

faster) alternatives in many cases, the use of subqueries introduces a hierarchy in execution that can be useful or necessary.

In the following example, the aggregation function A subquery can use values from the outer query, in which case it is known as a correlated subquery.

) is a special-purpose programming language designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS).

Originally based upon relational algebra and tuple relational calculus, SQL consists of a data definition language, data manipulation language, and Data Control Language.

The scope of SQL includes data insert, query, update and delete, schema creation and modification, and data access control.

Although SQL is often described as, and to a great extent is, a declarative language (4GL), it also includes procedural elements. introduced the first commercially available implementation of SQL, Oracle V2 (Version2) for VAX computers.

SQL was one of the first commercial languages for Edgar F. 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.

Codd's relational model, as described in his influential 1970 paper, "A Relational Model of Data for Large Shared Data Banks." Since then, the standard has been revised to include a larger set of features. SQL deviates in several ways from its theoretical foundation, the relational model and its tuple calculus.

Despite the existence of such standards, most SQL code is not completely portable among different database systems without adjustments. 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. Critics argue that SQL should be replaced with a language that strictly returns to the original foundation: for example, see The Third Manifesto.

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