On the Expressive Power of Node Construction in XQuery

the relational model it has been shown that the flat rela- tional algebra has the same expressive power as the nested relational algebra, as far as queries over flat relations and with flat results are concerned (6). Hence, for each query that uses the nested relational model and that, with a flat table as input always has a flat table as output, there exists an equivalent flat query that only uses the flat relational model. In analogy, we study a related flat-flat problem for XQuery: for each expression containing operations that con- struct new nodes and whose XML result contains only orig- inal nodes, there exists an equivalent "flat" expression in XQuery that does not construct new nodes.