storing difference Value

Question

hi expert, i have a probelm , &nbsp;i have table purcahseline which have column as <table border="0" cellspacing="2" cellpadding="2" width="500"><tbody><tr><td>&nbsp;PoId</td><td>&nbsp;Qty&nbsp;&nbsp;&nbsp;</td><td>&nbsp;company&nbsp;&nbsp;&nbsp;</td><td>&nbsp;Itemid</td><td>&nbsp;ItemName</td></tr></tbody></table>&nbsp;having multipel records, now when i used update query on this table, suppose it have old record<table border="0" cellspacing="2" cellpadding="2" width="500"><tbody><tr><td>&nbsp;PoId</td><td>&nbsp;Qty</td><td>&nbsp;Company</td><td>&nbsp;itemId</td><td>&nbsp;Qty</td></tr><tr><td>&nbsp;01</td><td>&nbsp;20</td><td>&nbsp;aa</td><td>&nbsp;ac-00002</td><td>&nbsp;Potato</td></tr></tbody></table><div><table border="0" cellspacing="2" cellpadding="2" width="500"><tbody><tr><td><br /></td><td><br /></td><td><br /></td><td><br /></td><td><br /></td></tr></tbody></table></div><div>when i update the record (qty) from 20 to 10 then there add a new field in same table which show me difference between old value and updated value.</div><div>&nbsp;</div>