SELECT
  r.Id AS RequestId,
  r.OpeningDate,
  rh.Note
FROM Requests r
INNER JOIN (SELECT
    *
  FROM RequestHistories rh
  WHERE rh.Id IN (SELECT MAX(_rh.Id) AS Id FROM RequestHistories _rh GROUP BY _rh.RequestId)) rh
  ON r.Id = rh.RequestId
INNER JOIN RequestHistoryPerformers rhp
  ON rh.Id = rhp.RequestHistoryId
WHERE r.ClosingDate IS NULL

Select
  r.Id as RequestId
  , r.OpeningDate
  , (select _p.LastName + ',' from performers _p where _p.Id=rhp.PerformerId for xml path('')) as Performers

SELECT
  STUFF((
    SELECT ', ' + [LastName]
    FROM performers
    FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)'),1,2,'')