sql - 递归查询列中的锚点和递归部分的类型不匹配

Question

给定一个类别@categoryId，查询应该递归地导航到最上面的超类别，这已经完成。

现在我还想生成一个字符串，这将是该过程中所有 CategoryName 的串联。

DECLARE @CategoryId AS int = 217;
WITH Categories AS
(
   SELECT ParentCategoryId, CategoryName, '' AS strCategory
   FROM Category 
   WHERE CategoryId = @CategoryId

   UNION ALL

   SELECT c.ParentCategoryId, c.CategoryName,
       (c.CategoryName + ': ' + cts.strCategory) AS strCategory  
   FROM Category AS c
   JOIN Categories AS cts
   ON c.CategoryId = cts.ParentCategoryId
)

SELECT TOP 1 CategoryName, LEN(CategoryName) AS strLength
FROM Categories
ORDER BY strLength DESC

使用上面的代码，我收到以下错误：

Types don't match between the anchor and the recursive part in column 
"strCategory" of recursive query "Categories".

感谢您的帮助

Answer 1

尝试更改查询以将 varchars 转换为 VARCHAR(MAX)。

就像是

DECLARE @CategoryId AS int = 217;
WITH Categories AS
(
   SELECT ParentCategoryId, CategoryName, CAST('' AS VARCHAR(MAX)) AS strCategory
   FROM Category 
   WHERE CategoryId = @CategoryId

   UNION ALL

   SELECT c.ParentCategoryId, c.CategoryName,
       CAST((c.CategoryName + ': ' + cts.strCategory) AS VARCHAR(MAX)) AS strCategory  
   FROM Category AS c
   JOIN Categories AS cts
   ON c.CategoryId = cts.ParentCategoryId
)

SELECT TOP 1 CategoryName, LEN(CategoryName) AS strLength
FROM Categories
ORDER BY strLength DESC