• 方式一

DELETE FROM t_test 
WHERE id IN 
(SELECT * FROM
 (SELECT MAX(id) did FROM t_test GROUP BY username HAVING COUNT(*) > 1) t);

• 方式二

DELETE FROM t_test 
WHERE id NOT IN 
(SELECT t.minno FROM 
 (SELECT MIN(id) AS minno FROM t_test GROUP BY username) t);

执行效率一优于二

MySQL 去重