UNION ALL 的问题

;WITH CTE AS
(
    SELECT [FolderID],[ParentID],CAST([FolderID] AS VARCHAR)+'\' AS [FolderPath]
    FROM [Folder] WHERE [FolderID]=28
    
    UNION ALL
    
    SELECT F.[FolderID],F.[ParentID],[FolderPath]+CAST(F.[FolderID] AS VARCHAR)+'\' FROM [Folder] AS F
    INNER JOIN CTE ON F.[ParentID]=CTE.[FolderID]
)
SELECT * FROM CTE

为什么这句sql执行不了，提示类型不匹配，但是我都转换为varchar了啊。我的[FolderID]是INT类型。
下面这样则可以执行成功,为什么？？

;WITH CTE AS
(
    SELECT [FolderID],[ParentID],CAST([FolderID] AS VARCHAR)+'\' AS [FolderPath]
    FROM [Folder] WHERE [FolderID]=28
    
    UNION ALL
    
    SELECT F.[FolderID],F.[ParentID],CAST(F.[FolderID] AS VARCHAR)+'\' FROM [Folder] AS F
    INNER JOIN CTE ON F.[ParentID]=CTE.[FolderID]
)
SELECT * FROM CTE

如果解决？？求解释。。。 SQL sql?server union?all 递归
[解决办法]

--以下：
;WITH CTE AS
(
    SELECT [FolderID],[ParentID],CAST(CAST([FolderID] AS VARCHAR)+'\'  AS VARCHAR) AS [FolderPath]
    FROM [Folder] WHERE [FolderID]=28
     
    UNION ALL
     
    SELECT F.[FolderID],F.[ParentID],CAST([FolderPath]+CAST(F.[FolderID] AS VARCHAR)+'\' AS VARCHAR) FROM [Folder] AS F
    INNER JOIN CTE ON F.[ParentID]=CTE.[FolderID]
)
SELECT * FROM CTE