select options in screens

Hello friends,

I am doing a screen program where the user will enter a range of sales orders,and i have to select from vbap the corresponding materials against that sales order number,but i am finding it difficult to use the select statement.

suppose my screen fields are vbeln-low and vbeln-high,so here i cant use the select statement select * from vbap into corresponding fields of iatb where

vbeln >= vbeln-low and vbeln <=vbeln-high, bcoz if the user doesnt enter vbeln-high then select statement goes wrong,other option i making a structure with fields like sign option low high,but if i have more select options that again beocmes a problem,is there anyway that i can solve this with a single select statement