SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

(No column name)
Microsoft SQL Server 2019 (RTM-CU16) (KB5011644) - 15.0.4223.1 (X64) Apr 11 2022 16:24:07 Copyright (C) 2019 Microsoft Corporation Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)

declare @t table (col1  int, col2 varchar(100))
insert into @t select 1, 'a'
insert into @t select 2, 'b'
insert into @t select 2, 'a'
insert into @t select 3, 'c'

declare @final_table table (col1  int, col2 varchar(100), col2_all varchar(1000))
insert into @final_table (col1, col2)
select * from @t

declare @col2_all varchar(1000)
declare @Name sysname

update  @final_table 
SET @col2_all = col2_all = COALESCE(CASE COALESCE(@Name, '') 
										WHEN col1 THEN @col2_all + '/' + col2
										ELSE col2 END, ''), 
									@Name = col1;

select col1, col2_grouped = MAX(col2_all)
from @final_table
group by col1

--Method 2
;with cte(col1,col2_grouped,rn)
as
(
  select col1, col2 , rn=ROW_NUMBER() over (PARTITION by col1 order by col1)
  from @t
)
,cte2(col1,final_grouped,rn)
as
(
select col1, convert(varchar(max),col2_grouped), 1 from cte where rn=1
union all
select cte2.col1, convert(varchar(max),cte2.final_grouped+'/'+cte.col2_grouped), cte2.rn+1
from cte2
inner join cte on cte.col1 = cte2.col1 and cte.rn=cte2.rn+1
)
select col1, MAX(final_grouped) col2_grouped from cte2 group by col1

col1	col2_grouped
1	a
2	b/a
3	c

col1	col2_grouped
1	a
2	b/a
3	c