SQL & PL/SQL :: Creating Trigger - Check If Data Being Inserted Already In Table And If Not Insert It

I know this is an old thread and I just started working with triggers. I want to check if the data being inserted is already in the table and if not insert it:

create or replace trigger t_triggername
before insert on tbl_tablename
for each row
begin
if(:new.user_id <> :old.user_id) then
insert into tbl_tablename(user_id, location)
values (:new.user_id, :new.location);
end if;
end;

what if I wanted to keep the user but only update location if the user is already in the table. I've tried doing it this way:

create or replace trigger t_triggername
before insert on tbl_tablename
for each row
begin
if(:new.user_id <> :old.user_id) then
insert into tbl_tablename(user_id, location)

[code]...