对于 Nvarchar(Max)，我在 TSQL 中只能获取 4000 个字符？

Question

这是 SS 2005 的。

为什么我只得到 4000 个字符而不是 8000 个？

它在 4000 处截断字符串 @SQL1。

ALTER PROCEDURE sp_AlloctionReport(
    @where NVARCHAR(1000),
    @alldate NVARCHAR(200),
    @alldateprevweek NVARCHAR(200))
AS
    DECLARE @SQL1 NVARCHAR(Max)

    SET @SQL1 = 'SELECT DISTINCT VenueInfo.VenueID, VenueInfo.VenueName, VenuePanels.PanelID, 
    VenueInfo.CompanyName, VenuePanels.ProductCode, VenuePanels.MF, VenueInfo.Address1, 
    VenueInfo.Address2, '' As AllocationDate, '' As AbbreviationCode, VenueInfo.Suburb, VenueInfo.Route, VenueInfo.ContactFirstName, 
    VenueInfo.ContactLastName, VenueInfo.SuitableTime, VenueInfo.OldVenueName, 
    VenueCategories.Category, VenueInfo.Phone, VenuePanels.Location, VenuePanels.Comment, 
    [VenueCategories].[Category] + '' Allocations'' AS ReportHeader, 
    ljs.AbbreviationCode AS PrevWeekCampaign
    FROM (((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID = VenuePanels.VenueID) 
    INNER JOIN VenueCategories ON VenueInfo.CategoryID = VenueCategories.CategoryID) 
    LEFT JOIN (SELECT CampaignProductions.AbbreviationCode, VenuePanels.PanelID, CampaignAllocations.AllocationDate
                    FROM (((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID=VenuePanels.VenueID) INNER JOIN CampaignAllocations ON VenuePanels.PanelID=CampaignAllocations.PanelID) INNER JOIN CampaignProductions ON CampaignAllocations.CampaignID=CampaignProductions.CampaignID) INNER JOIN VenueCategories ON VenueInfo.CategoryID=VenueCategories.CategoryID
                    WHERE ' + @alldateprevweek + ') ljs
                ON VenuePanels.PanelID = ljs.PanelID) 
    INNER JOIN (SELECT VenueInfo.VenueID, VenuePanels.PanelID, VenueInfo.VenueName, VenueInfo.CompanyName, VenuePanels.ProductCode, 
                VenuePanels.MF, VenueInfo.Address1, VenueInfo.Address2, CampaignAllocations.AllocationDate, 
                CampaignProductions.AbbreviationCode, VenueInfo.Suburb, VenueInfo.Route, VenueInfo.ContactFirstName, 
                VenueInfo.ContactLastName, VenueInfo.SuitableTime, VenueInfo.OldVenueName, VenueCategories.Category, 
                VenueInfo.Phone, VenuePanels.Location, VenuePanels.Comment, [Category] + '' Allocations'' AS ReportHeader, 
                ljs2.AbbreviationCode AS PrevWeekCampaign
                FROM ((((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID = VenuePanels.VenueID) 
                INNER JOIN CampaignAllocations ON VenuePanels.PanelID = CampaignAllocations.PanelID) 
                INNER JOIN CampaignProductions ON CampaignAllocations.CampaignID = CampaignProductions.CampaignID) 
                INNER JOIN VenueCategories ON VenueInfo.CategoryID = VenueCategories.CategoryID) 
                LEFT JOIN (SELECT CampaignProductions.AbbreviationCode, VenuePanels.PanelID, CampaignAllocations.AllocationDate
                                FROM (((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID=VenuePanels.VenueID) INNER JOIN CampaignAllocations ON VenuePanels.PanelID=CampaignAllocations.PanelID) INNER JOIN CampaignProductions ON CampaignAllocations.CampaignID=CampaignProductions.CampaignID) INNER JOIN VenueCategories ON VenueInfo.CategoryID=VenueCategories.CategoryID
                                WHERE ' + @alldateprevweek + ') ljs2
                            ON VenuePanels.PanelID = ljs2.PanelID
                WHERE ' + @alldate + ' AND ' + @where + ') ljs3
                ON VenueInfo.VenueID = ljs3.VenueID
    WHERE (((VenuePanels.PanelID)<>ljs3.[PanelID] And 
        (VenuePanels.PanelID) Not In (SELECT PanelID FROM CampaignAllocations WHERE ' + @alldateprevweek + ')) 
        AND ' + @where + ')
    UNION ALL
     SELECT VenueInfo.VenueID, VenueInfo.VenueName, VenuePanels.PanelID, VenueInfo.CompanyName, VenuePanels.ProductCode, 
    VenuePanels.MF, VenueInfo.Address1, VenueInfo.Address2, CampaignAllocations.AllocationDate, 
    CampaignProductions.AbbreviationCode, VenueInfo.Suburb, VenueInfo.Route, VenueInfo.ContactFirstName, 
    VenueInfo.ContactLastName, VenueInfo.SuitableTime, VenueInfo.OldVenueName, VenueCategories.Category, 
    VenueInfo.Phone, VenuePanels.Location, VenuePanels.Comment, [Category] + '' Allocations'' AS ReportHeader, 
    ljs.AbbreviationCode AS PrevWeekCampaign
    FROM ((((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID = VenuePanels.VenueID) 
    INNER JOIN CampaignAllocations ON VenuePanels.PanelID = CampaignAllocations.PanelID) 
    INNER JOIN CampaignProductions ON CampaignAllocations.CampaignID = CampaignProductions.CampaignID) 
    INNER JOIN VenueCategories ON VenueInfo.CategoryID = VenueCategories.CategoryID) 
    LEFT JOIN (SELECT CampaignProductions.AbbreviationCode, VenuePanels.PanelID, CampaignAllocations.AllocationDate
                    FROM (((VenueInfo INNER JOIN VenuePanels ON VenueInfo.VenueID=VenuePanels.VenueID) INNER JOIN CampaignAllocations ON VenuePanels.PanelID=CampaignAllocations.PanelID) INNER JOIN CampaignProductions ON CampaignAllocations.CampaignID=CampaignProductions.CampaignID) INNER JOIN VenueCategories ON VenueInfo.CategoryID=VenueCategories.CategoryID
                    WHERE ' + @alldateprevweek + ') ljs
                ON VenuePanels.PanelID = ljs.PanelID
    WHERE ' + @alldate + ' AND ' + @where

    Select @SQL1

Answer 1

您已将其声明为 nvarchar(max)，它允许 2GB 的数据，因此它将存储 2GB。

发生了什么：

• 在分配给@sql1之前，数据类型还不是nvarchar(max)，
• 在此之前，它是一个字符串集合，每个字符串小于4000 (constants)
• 您将短常量与短变量连接起来 (short = < 4000)
• 所以您有 4000放入 @sql1 的字符

因此，您必须确保右侧有 nvarchar(max) 。

一个想法。第二行将 nvarchar(max) 与常量 = nvarchar(max) 连接起来，

SET @SQL1 = ''
SET @SQL1 = @SQL1 + 'SELECT DISTINCT Venue...
   ....

这与每种语言中发生的整数除法没有什么不同。

declare @myvar float
set @myvar = 1/2 --gives zero because it's integer on the right

运算符优先级（推断数据类型优先级）始终是“赋值”最后...为什么 SQL Server 中的 unicode 字符串应该有所不同？

Answer 2

更新：gbn的答案是对的，而我错了。正如 MSDN 指出的，nvarchar(max) 最多支持 2^ 31-1 个字节的数据，存储为 UCS-2（每个字符 2 个字节，加上 2 个 BOM）。您的问题似乎与字符串连接有关，而不是数据类型限制。

也就是说，如果您使用它来构建 SQL 字符串，为什么不使用 VARCHAR？您是否有无法用数据库的本机字符集（通常是 Latin-1）表示的字段名称？

最后——您可以通过在存储过程中不使用动态 SQL 来简化整个问题。创建一些表值函数来获取 where 子句字符串并返回表，然后将它们连接到您的过程中。作为奖励，它几乎肯定会快得多，因为至少数据库将能够将 SP 主体缓存为准备好的语句。

Answer 3

我解决问题
只需在每个字符串之前添加 N 个字符即可解决问题
例如，

declare @sql nvarchar(max) = '' + @Where + 'SomeThing';

如果

declare @sql nvarchar(max) = N'' + @Where + N'SomeThing';

您将字符串设置为空，也必须设置 N''

if @where is null
set @where = N''

:-) 简单的答案