UNION ALL 的问题

[ZXY900213]

;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。。。

[lzw_0736]

--試試以下：
;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

[ZXY900213]

引用 1 楼 lzw_0736 的回复:

--試試以下：
;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

果然可以了，，为什么varchar类型相加，数据类型会变呢？？

[lzw_0736]

原因我也不清楚，只是實際使用過程中偶然發現的規則.