A formal basis for extending SQL to object-oriented databases.
Jan Van den Bussche · 1990
A formal basis for extending SQL in a natural way to complex object databases is laid. NSQL, a language equivalent to the standard algebra for nested relational databases, is obtained. It is also shown that, when allowing subqueries in the from-clause, the facility of subqueries in where-clauses becomes redundant with regard to expressive power.