At first I thought you could do this with window functions (http://www.postgresql.org/docs/9.1/static/tutorial-window.html http://www.postgresql.org/docs/9.1/st

At first I thought you could do this with window functions (http://www.postgresql.org/docs/9.1/static/tutorial-window.html http://www.postgresql.org/docs/9.1/static/tutorial-window.ht...) but now I'm not so sure, or at least it doesn't seem very straightforward to me - very neat!