WITH treeCustomers( ID, ParentID, Description, Level) AS
(
  SELECT id, ParentID, Description, 1
    FROM [refCustomers_Groups]
    WHERE ParentID = 0
  UNION ALL
  SELECT t1.id, t1.ParentID, t1.Description, Level + 1
    FROM [refCustomers_Groups] as t1
      INNER JOIN treeCustomers on t1.ParentID = treeCustomers.ID 
    WHERE  t1.ParentID <> 0)
SELECT  ID, ParentID, Description, Level
  FROM treeCustomers