select count(f_uid) num, f_uid from t_crm_user 
group by f_uid having count(f_uid) > 1
order by num desc

DELETE FROM t_crm_user where f_uid IN (
    SELECT f_uid from (
        SELECT f_uid FROM t_crm_user GROUP BY f_uid HAVING count(f_uid) > 1
    ) a
) AND
 
 id NOT IN (
    SELECT keepId FROM (
        SELECT max(id) keepId FROM t_crm_user GROUP BY f_uid HAVING count(f_uid) > 1
    ) b
)

-- 效率比較慢
select a.* 
from
(
select t1.*,(select count(*)+1 from 表 where 分組欄位=t1.分組欄位 and 排序欄位<t1.排序欄位) as group_id
from 表 t1
) a
where a.
 
group_id<=3

SELECT t.*         
   FROM (SELECT ROW_NUMBER() OVER(PARTITION BY 分組欄位 ORDER BY 排序欄位 DESC) rn,         
         b.*         
         FROM 表 b) t         
  WHERE t.rn <= 3  ;