1. A method for converting data between eXtensible Markup Language (XML) constructs and Structured Query Language (SQL) constructs in an SQL compliant database management system (DBMS) that allows instances of XML type to represent XML constructs, comprising the steps of: receiving a SQL statement that includes a particular operator that operates on a first instance of XML type that represents a first set of zero or more XML elements; and during execution of the SQL statement, evaluating the particular operator by generating an ordered collection of zero or more instances of XML type, wherein each different instance of XML type in the ordered collection is based on a different XML element from the first set; and there is an instance of XML type in the ordered collection for every XML element in the first set.