According to [Immerman][1], the complexity class associated with [SQL][2] queries is exactly the class of *safe queries* in $\mathsf{Q(FO(COUNT))}$ (first-order queries plus counting operator). 

Is there an extension of SQL (implemented and used in the industry) which captures polynomial time computable queries (and no others)? If there isn't, is there a reason? Are the queries that are needed in practice usually simple enough that there is no need for a stronger language?

  [1]: http://books.google.ca/books?id=kWSZ0OWnupkC&pg=PA224#v=onepage&q&f=false
  [2]: http://en.wikipedia.org/wiki/SQL