MySQL 8.0

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CREATE TABLE t (
  Id INTEGER,
  Membership_id INTEGER,
  promocode VARCHAR(5),
  createdtime date
);

INSERT INTO t
  (Id, Membership_id, promocode, createdtime)
VALUES
  ('1', '1', 'New10', '2022-08-01'),
  ('2', '1', 'New20', '2022-09-14'),
  ('3', '1', null, '2022-09-25');            

Records: 3  Duplicates: 0  Warnings: 0


select * from t


Id
Membership_id
promocode
createdtime


1
1
New10
2022-08-01

2
1
New20
2022-09-14

3
1
null
2022-09-25


select   max(id)          as id
        ,Membership_id   
        ,max(promocode)   as promocode
        ,max(createdtime) as createdtime
from     t
group by Membership_id


id
Membership_id
promocode
createdtime


3
1
New20
2022-09-25


select   id
        ,Membership_id       
        ,first_promocode as promocode
        ,createdtime
from     (
         select  *
                ,max(case when grp = 1 then promocode end) over(partition by Membership_id order by createdtime) as first_promocode
         from    (
                 select  *
                         ,count(case when promocode is not null then 1 end) over(partition by Membership_id order by createdtime) as grp
                         ,rank() over(partition by Membership_id order by createdtime desc) as rnk
                  from    t
                 ) t
         ) t
where    rnk = 1


id
Membership_id
promocode
createdtime


3
1
New10
2022-09-25