[MorenoArdohain]

Unsigned polje u mysql-u nikad ne moze biti negativan, a pretpostavljam da je i rezultat "unsigned" pa je doslo do konverzije vrednosti u max vrednost za int.

Edit: mozda se nisam dobro izrazio, evo u manualu:
http://dev.mysql.com/doc/refman/5.0/...functions.html i http://dev.mysql.com/doc/refman/5.0/...-overflow.html

If both operands are integers and any of them are unsigned, the result is an unsigned integer.

Subtraction between integer values, where one is of type UNSIGNED, produces an unsigned result by default. If the result would otherwise have been negative, it becomes the maximum integer value.