Hi Srikant,
i think whatever Arun is sujjesting is right, but please change your code as shown below
@K.Arun Prabhu, i think he has to use if it_final[] is initial, isn't it.
WHEN 'ENTER'.
PERFORM DISP_DATA.
FORM DISP_DATA .
IF IT_FINAL[] IS INITIAL.
SELECT MANDT FIELD1 FIELD2
FROM ZTABLE
INTO TABLE IT_FINAL.
ENDIF.
ENDFORM.