偶尔和同事聊到这个问题就顺便写了下代码，供大家参考~
//数据准备
drop table t_user;
create table t_user(
id int(5) not null auto_increment,
username varchar(10),
age int(3),
primary key(id)
);
insert into t_user(username,age) values('aaa',20);
insert into t_user(username,age) values('aaa',20);
insert into t_user(username,age) values('aaa',20);
insert into t_user(username,age) values('bbb',20);
insert into t_user(username,age) values('bbb',20);
insert into t_user(username,age) values('ccc',20);
insert into t_user(username,age) values('ccc',20);
insert into t_user(username,age) values('ddd',20);
insert into t_user(username,age) values('ddd',20);
删除语句：
DELETE t
FROM
t_user t,
(
SELECT
min(id)AS ttid,
username
FROM
t_user t2
GROUP BY
t2.username
)AS tt
WHERE t.id > tt.ttid
and t.username = tt.username;