Try as below
select c.id
, c.bereichsname
, STRING_AGG( CAST(j.oberbereich as nvarchar(MAX)),',') oberBereiches
from stellenangebote_archiv j
join bereiche c on j.bereich_id = c.id
group by c.id, c.bereichsname
So the problem is the length of the concatenated string is exceeding the character limit of the result column.
So we are setting the limit to max by converting all values to “nvarchar(max)” to solve the problem.
And “STRING_AGG()” function returns what it gets.