Seems like at least some SQL-like flexibility could be added by using custom keywords in the Accept header. There's an example of using `depth` parameter for specifying how far server should dig into related resources[1].

Surely other keywords could be added as well (e.g., specifying a set of resource fields, ordering, etc). A RESTful solution, although probably not quite elegant.

[1] http://www.stereoplex.com/blog/mobile-api-design-thinking-be...