ListAGG in SQLSERVER

Starting in SQL Server 2017 the STRING_AGG function is available which simplifies the logic considerably:

select FieldA, string_agg(FieldB, '') as data
from yourtable
group by FieldA

See SQL Fiddle with Demo

In SQL Server you can use FOR XML PATH to get the result:

select distinct t1.FieldA,
  STUFF((SELECT distinct '' + t2.FieldB
         from yourtable t2
         where t1.FieldA = t2.FieldA
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,0,'') data
from yourtable t1;

See SQL Fiddle with Demo