How to check previous quantity from previous table ?

hello dear
I am in problems to check previous quantity from previous table when i input new quantity
its must me less than or equal previous table qty.
my trial is below
This is my PR table. My PR qty is 100,

	Type	source		Desc 1	Desc 2	Quantity	Required Quantity		Unit	Accsl
	Accessories	p1		somthing rong	good	100			KG	14

This is my WO table

	Type	source	source	Desc 1	Desc 2		Required Quantity		Unit	Accsl
	Accessories	W1	i1	somthing rong	good				KG	14

If i want to insert >100 in WO table programe prevent to insert how can i do it
if anybody help me best regurds.