group_concat in sql-server, Sql Server

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CREATE table #tmp (Nummer int, Betreff varchar(32) null, Datum date null );

INSERT INTO #tmp
          SELECT 1,'2.000 EUR Rente','2017-03-06'
UNION ALL SELECT 1,'2.000 EUR Rente',NULL
UNION ALL SELECT 1,'2.000 Rente','2017-03-04'
UNION ALL SELECT 1,'2.000 Rente','2017-02-02'
UNION ALL SELECT 1,'2.000 Rente','2017-03-12'
UNION ALL SELECT 1,'2.000 Rente','2017-02-19'
UNION ALL SELECT 1,'2.000 Rente','2017-03-19'
UNION ALL SELECT 1,'2.000 Rente','2017-02-27'
UNION ALL SELECT 1,'2.000 Rente',NULL
UNION ALL SELECT 1,'2.000 Rente','2017-02-27'
UNION ALL SELECT 1,'2.000 Rente',NULL
UNION ALL SELECT 2,'2.000 EUR Rente','2017-03-06'
UNION ALL SELECT 2,'2.000 EUR Rente',NULL
UNION ALL SELECT 2,'2.000 Rente','2017-03-04'
UNION ALL SELECT 2,'2.000 Rente','2017-02-12'
UNION ALL SELECT 2,'2.000 Rente','2017-03-12'
UNION ALL SELECT 2,'2.000 Rente','2017-02-19'
UNION ALL SELECT 2,'2.000 Rente','2017-03-29'
UNION ALL SELECT 2,'2.000 Rente','2017-02-27'
UNION ALL SELECT 2,'2.000 Rente',NULL
UNION ALL SELECT 2,'2.000 Rente','2017-02-27'
UNION ALL SELECT 2,'2.000 Rente',NULL;

SELECT Nummer,min(Datum) miDat, max(Datum) maDat,
       STUFF( (SELECT ','+char(10) + CONVERT(char(10),Datum,104) FROM #tmp 
               WHERE NOT Datum is null AND Nummer=o.Nummer
               ORDER BY Datum
               FOR XML PATH('')), 1, 2, '' ) as dates 
FROM #tmp o GROUP BY Nummer

drop table #tmp

View schema
Execution time: 0,03 sec, rows selected: 2, rows affected: 22, absolute service time: 0,19 sec