At selection-screen help req?

hi

I had a filed p_doc as parameter.

I need that,if its blank,the itab shd be filled with all the doc type(except 'NB',coz for it WHERE conditionds are differnt means different select query),if its put with NB doc type,it shd only pick the data for it.I had made the perfors for both selct queries,but unable to handle this at AT Selection-Screen.

I am coding:

at selction-screen on p_doc.

if p_doc = 'NB'. -

( in debugging,i found- p_doc = IENB)

Perform one.

else perform two.

endif.

so for doc type NB its going to nxt statemnt.What shd i do.

plz help me??

Rgds

Vipin