selection queries

Hi all,

i have 7 inputs in selection screen from the 3 difrnt tables (4from one table and 2 from one table and 1 from one table.). and i want to fetch data in difrnt combinations.and in start of selection i m writing condition like below. but it is not performaing.there is no mandatory field. can anybody give me short way to fetch data from difrnt combinations. very urgent....

im writing the if condition like this. for two combinations.

START-OF-SELECTION.

IF S_WERKS EQ ' ' AND S_MATNR EQ ' ' AND S_LIFNR EQ ' ' AND S_MTART EQ 'X' OR S_MATKL EQ 'X'.

PERFORM GET_MARA.

ELSEIF S_WERKS EQ 'X' OR S_MATNR EQ 'X' OR S_LIFNR EQ 'X'.

PERFORM GET_MSEG.

ENDIF.