UPDATE returns the number of rows that were actually changed. In MySQL 3.22 or later, the mysql_info() C API function returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

"UPDATE ritorna il numero di righe che sono state cambiate etc etc..."

preso paro paro dal sito di mysql:
http://dev.mysql.com/doc/mysql/en/UPDATE.html