How to calculate data within 2 tables?

Question

my database:

PRODUCT ORDER

======= =========

ID_PRODUCT ID_ORDER

NAME ID_PRODUCT

P_QTY O_QTY

how to calculate the QTY?

In ORDER form i have txtO_QTY, and btnSUBMIT, 

if i submit the button, the P_QTY on PRODUCT table will decreased depends on the value of O_QTY.

thx in advance