Form question.......

Hi, i have a question, it may be dumb, but i´m very new with php and mysql, i need help. how can i check a form when i change from one field to another???? I want make a condition.....if the number i typed is allready in the database, not just when i finish filling out the hole form...clickin in submit and then make the conditions....help !!!! heheheh -------------------------------------------------------------------------------- Rodrigo Corrêa rodrigo@praticsistemas.com.br Obs: Sorry about the english, i´m from Brazil...so my english isn´t good