Journal ArticleSQL is a relational database definition and manipulation language. Portions of the manipulation language are readily described in terms of relational algebra. The semantics of a subset of the SQL select statement is described. The select statement allows the user to query the database. The select statement is shown to be equivalent to a series of relational and set operations. The semantics are described in terms of abstract data types for relation schemes, tuples, and relations. Certain forms of the union or intersection of two select statements are shown to have equivalent single select statement forms
