删除重复数据保留id最大那条

select b.id,b.username
  from test b
 where b.id not in (select max(t.id) as id
                      from test t
                    having count(t.username) > 1
                     group by t.username)
   and b.username in (select t.username
                  from test t
                having count(t.username) > 1
                 group by t.username);