09 September 2009

Basic Structure of SQL

The basic structure of an SQL expression consists of three clauses:
  • The select clause corresponds to the projection operation of the relational algebra. It is used to list the attributes desired in the result of a query.
  • The from clause corresponds to the Cartesian-product operation of the relational algebra. It lists the relations to be scanned in the evaluation of the expression.
  • The where clause corresponds to the selection predicate of the relational algebra. It consists of a predicate involving attributes of the relations that appear in the from clause.

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