Ignoring empty select-option inputs

Question

Hi all,
Have 3 input select options as follows:
select-options: so_retnr for vbak-vbeln.
select-options: so_matnr for mara-matnr.
select-options: so_bstnr for vbak-bstnk.
Iam using the above 3 select options in the following select.
Now the user can fill-in all the above select inputs or any 1 or 2 of them.
In my select statement I want to use only those select-options filled in by the user so that it wont make a full table search for the empty select-option input.
Is there any easy way to check that in the select statement.
  select vbakvbeln vbakbstnk vbakihrez vbakaudat vbak~auart
       vbapmatnr vbaparktx vbap~kwmeng into corresponding fields
       of table gt_zrritab from vbak inner join vbap on
        vbakvbeln = vbapvbeln
        where  vbak~vbeln in so_retnr and
               vbap~matnr in so_matnr and
                vbak~bstnk in so_bstnr and
                vbak~auart = 'ZRR'.
Regards
P

Answer 1

data: t_string type string.
if not so_retnr[] is initial and not so_matnr[] is initial and not so_bstnr[] is initial.
  t_string = 'vbakvbeln IN so_retnr AND vbapmatnr IN so_matnr AND vbak~bstnk IN so_bstnr'.
elseif not so_retnr[] is initial and not so_matnr[] is initial and so_bstnr[] is initial
t_string = 'vbakvbeln IN so_retnr AND vbapmatnr IN so_matnr.
elseif not so_retnr[] is initial and so_matnr[] is initial and not so_bstnr[] is initial
  t_string = 'vbakvbeln IN so_retnr AND vbakbstnk IN so_bstnr'.
elseif.
etc....
endif.
  SELECT vbakvbeln vbakbstnk vbakihrez vbakaudat vbak~auart
  vbapmatnr vbaparktx vbap~kwmeng INTO CORRESPONDING FIELDS
  OF TABLE gt_zrritab FROM vbak INNER JOIN vbap ON
  vbakvbeln = vbapvbeln
  WHERE (t_string)
   AND vbak~auart = 'ZRR'.
zashok