Module pool Program

Hi,

This is my module pool program.

When iam executing this using t.code data of the customer was not retrieved.

Plz let me know where i had gone wrong.

MODULE USER_COMMAND_9001 INPUT.

TABLES:

KNA1.

CASE SY-UCOMM.

WHEN 'EXIT'.

LEAVE PROGRAM.

WHEN 'DISPLAY'.

IF KNA1-KUNNR IS NOT INITIAL.

SELECT SINGLE * FROM KNA1 WHERE KUNNR EQ KNA1-KUNNR.

ENDIF.

IF SY-SUBRC NE 0.

MESSAGE E000(0) WITH 'CUSTOMER NUMBER INVALID'.

ELSE.

MESSAGE E000(0) WITH 'INPUT CUSTOMER NUMBER'.

ENDIF.

WHEN 'INSERT'.

INSERT KNA1.

IF SY-SUBRC NE 0.

MESSAGE E000(0) WITH 'RECORD INSERTED'.

ENDIF.

ENDCASE.

ENDMODULE. " USER_COMMAND_9001 INPUT