SQL Server 2019

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select *
into datas
from (values ('a',  3  , 'sd87'),
     ('a', null, 'sd87'),
     ('a', null, 'sd87'),
     ('a', 5   , 'cz6'),
     ('b', null, '87'),
     ('b', null, '87'),
     ('b', null, '87'),
     ('d', 22  , 'ah54'),
     ('d', null, 'ah54'),
     ('d', null, 'fr45'),
     ('d', 21  , 'ah54')
) v (id, caseid, userid)
11 rows affected

select id, max(caseId) as caseId, UserId
from datas
group by id, UserId;


id
caseId
UserId


b
null
87

d
22
ah54

a
5
cz6

d
null
fr45

a
3
sd87


Warning: Null value is eliminated by an aggregate or other SET operation.


select distinct d.*
from datas d
where d.caseId is not null or
      not exists (select 1 from datas d2 where d2.userid = d.userid and d2.caseid is not null)


id
caseid
userid


a
3
sd87

a
5
cz6

b
null
87

d
null
fr45

d
21
ah54

d
22
ah54