Group by XML PATH

Group_By_XML_Path.sql

 
with partitioned_data as
(
	select dossier, dos from (
	  select dossier, ntile(50) over(order by dossier) as dos from [dbo].[input_legacy_excel]
	) x

)
Select P.dos, SUBSTRING(STUFF(
(
	SELECT ',' + dossier
	FROM partitioned_data
	where dos = p.dos
	FOR XML PATH(''),TYPE).value('(./text())[1]', 'VARCHAR(MAX)'), 1,2,''
), 2, 999999) As dossiers
from partitioned_data p
group by p.dos