Well, I'm not the judge and I don't know what other solutions will be sent in, but I guess it's hard to beat this magnificent O(NlogN) solution to the First International NoCOUG SQL Challenge by Alberto Dell'Era, posted here on his website. If you have a statistics or mathematics background, you might actually be able to understand it completely. If not or if you forgot most of it, like me, then just watch and enjoy. Who said SQL is not complete? :-)
Hats off and a deep bow to you, Alberto.