We study the problem of deciding satisfiability of first order logic
queries over views, our aim being to delimit the boundary between
the decidable and the undecidable fragments of this language.
 Views currently
occupy a central place in database research, due to their role in
applications such as information integration and
data warehousing.
 Our principal result is the identification
of an important decidable class of queries over unary conjunctive views.
This extends the decidability of the classical class
of first order sentences over unary relations (the L\"{o}wenheim class). 
We then demonstrate how extending this class leads to undecidability. 
In addition to new areas,
our work
also has relevance to extensions of results for related problems such as 
query containment, trigger termination, 
implication of dependencies and reasoning in description logics.