  The conservativity theorem for nested relational calculus implies that query
expressions can freely use nesting and unnesting, yet as long as the query
result type is a flat relation, these capabilities do not lead to an increase
in expressiveness over flat relational queries. Moreover, Wong showed how such
queries can be translated to SQL via a constructive rewriting algorithm. While
this result holds for queries over either set or multiset semantics, to the
best of our knowledge, the questions of conservativity and normalization have
not been studied for queries that mix set and bag collections, or provide
duplicate-elimination operations such as SQL's
$\mathtt{SELECT}~\mathtt{DISTINCT}$. In this paper we formalize the problem,
and present partial progress: specifically, we introduce a calculus with both
set and multiset collection types, along with natural mappings from sets to
bags and vice versa, present a set of valid rewrite rules for normalizing such
queries, and give an inductive characterization of a set of queries whose
normal forms can be translated to SQL. We also consider examples that do not
appear straightforward to translate to SQL, illustrating that the relative
expressiveness of flat and nested queries with mixed set and multiset semantics
remains an open question.
