SQL & PL/SQL :: Procedure To Display Data From Drop Down List

I have to write a proc to display data based on user input for t_grp, and it should exclude grp0

So below is the proc I have written, and I want to display the report for all t_grp when user gives user_input as All or gives null. How can I acheieve this is in procedure.

table: target_data

create table target_data
(
t_mon varcahr2(6)
, t_grp varchar2(30)
, t_data varchar2(50)
)
;
INSERT INTO target_data VALUES ('Apr-11', 'grp1', 'some_data');
INSERT INTO target_data VALUES ('Apr-11', 'grp2', 'some_data');
INSERT INTO target_data VALUES ('Apr-11', 'grp4', 'some_data');
[code]......

Now the above proc is running fine, if I give the input as grp1 or any other t_grp.