create table test_members (
username varchar(20),
referral varchar(20));

insert into test_members values ('Jack',null);
insert into test_members values ('Bob','Jack');
insert into test_members values ('Joe','Bob');
insert into test_members values ('Henry','Joe');
insert into test_members values ('Gabe','Jack');
insert into test_members values ('Pete','Gabe');
insert into test_members values ('Tim','Gabe');