VARCHAR(MAX)在连接字符串时表现得很奇怪

Question

我有一个连接查询:

DECLARE @path NVARCHAR(max) 
SELECT @path = ISNULL(@path + '/', '') + url_segment 
    FROM navigation_self_and_parents(2813) ORDER BY depth ASC
SELECT @path

navigation_self_and_parents(2813) 回报

id    par_id  title        url_segment    sequence        depth
2813  2816    testing1234  testing1234    0               0   
2816  2809    U            /fixedurl      0               -1   
2809  NULL    E            E              0               -2   

我的串联查询返回

'testing1234'             when using `NVARCHAR(MAX)` and
'E//fixedurl/testing1234' when using `NVARCHAR(4000)`

我最好的猜测是使用NVARCHAR(MAX)原因@path每次设置都要重新输入,因此在重新输入之前丢失了设置的内容,或者在第一次设置时输入,然后后续连接调用无声地失败.

我很想真正理解这种行为的根本原因.

UPDATE

navigation_self_and_parents:

USE [SomeDatabase]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[navigation_self_and_parents]
(   
    @id int
)
RETURNS TABLE 
AS
RETURN 
(
    WITH navigation_self_and_parents (id, parent_id, title, url_segment, sequence_number, depth)
    AS
    (
        SELECT id, parent_id, title, url_segment, sequence_number, 0 FROM navigation_node WHERE id=@id 
        UNION ALL

        SELECT n.id, n.parent_id, n.title, n.url_segment, n.sequence_number, depth - 1 From navigation_node as n
        INNER JOIN navigation_self_and_parents as rn
        ON n.id = rn.parent_id
    )
    SELECT * FROM navigation_self_and_parents
)

navigation_node DDL:

CREATE TABLE [dbo].[navigation_node](
    [id] [int] IDENTITY(1,1) NOT NULL,
    [title] [nvarchar](128) NULL,
    [url_segment] [nvarchar](max) NULL,
    [hidden] [bit] NOT NULL,
    [page_id] [int] NULL,
    [parent_id] [int] NULL,
    [sequence_number] [int] NOT NULL,
    [createdOn] [datetime] NOT NULL,
    [updatedOn] [datetime] NULL,
    [navigation_type_id] [int] NULL,
    ...snap

Answer 1

这种字符串连接方法通常有效，但不能保证。

知识库文章中针对类似问题的官方说法是“聚合串联查询的正确行为未定义”。

计划之间肯定存在一些细微的差异。您可以调整查询以消除差异并获得所需的执行计划，或者您当然可以/应该XML PATH像在 SQL Server 2005 上一样使用，并且这已被记录为有效。