참고요 

with userlist as (
    select 'A0001' as sabun,'AAAAAA' as fullname, '010-1111-2222' as celno,    'A123123123.jpg' as sajin
    union all select 'A0002', 'BBBBBB', '010-2222-3333',   'BD3423423.jpg'
    union all select 'A0003', 'CCCCCC', '010-4444-3333',   'BD333333.jpg'
    union all select 'A0004', 'DDDDDD', '010-6666-3333',   'B4444444.jpg')
, toto_list as (
    select 1 as idx, '수강신청 마무리 하기' as subject, 'A0001' as writeid, 'A0002,A0003' as shareid
    union all select 2, '학생회비 납부 확인하기', 'A0001',  'A0002,A0003,A0004'
)
select
    x.idx,
    x.subject,
    x.writeid,
    string_agg(x.shareid, ',') as shareid,
    string_agg(y.sajin, ',') as sajin
from (select idx,
             subject,
             writeid,
             b.shareid
      from toto_list a
               cross apply (select value as shareid from string_split(a.shareid, ',')) b) x
inner join userlist y
on  x.shareid = y.sabun
group by x.idx, x.subject, x.writeid