SQL Server 2017

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CREATE TABLE Table1
    (ID int, USERs varchar(1), BDATE date)
INSERT INTO Table1
    (ID, USERs, BDATE)
VALUES
    (1, 'A', '2018-10-01'),
    (2, 'A', '2018-09-01'),
    (3, 'A', NULL),
    (4, 'B', '2018-05-03'),
    (5, 'B', '2017-04-01'),
    (6, 'C', NULL)
;
6 rows affected

select * 
    from
    (select *, row_number() over(partition by users order by users,case when bdate is null then 0 else 1 end desc,bdate) as rn
from Table1)x where rn=1


ID
USERs
BDATE
rn


2
A
01/09/2018 00:00:00
1

5
B
01/04/2017 00:00:00
1

6
C
null
1