SQL触发器: Change nvarchar大小

我有几个带有nvarchar(max)列的表格。我正在构建一个触发器，尝试将单元格存储在sql_Variant中。nvarchar(max)无法存储在sql_variant中。我如何检查它是否为nvarchar(max)，然后将其转换为nvarchar(4000)，以避免出现这个问题？

编辑：最可能的错误发生在这里：

set @sql = 'select @audit_oldvalue=[' +@Item +'] from #tempTrigT';

因为@audit_oldvalue是sql_variant，而[' +@Item +'] from #tempTrigT可以是nvarchar(max)。

如果我在输入表中进行编辑，我会得到这个错误：

操作数类型冲突，nvarchar(max)与sql_variant不兼容

表格结构：

输入表

输出表

我想要转换的代码：

Select * into #tempTrigT from (select * from deleted where @Action in ( 'U','D')) A UNION (select * from inserted where @Action ='I') 
Select @sql = @sql + 'Case when IsNull(i.[' + Column_Name +  
'],0) = IsNull(d.[' + Column_name + '],0) then ''''  
 else ' + quotename(Column_Name, char(39)) + ' + '',''' + ' end +' 
from information_schema.columns  
where table_name = 'Forms' and column_name <>'rowguid' and column_name <>'modifieddate'
--定义输出参数 
set @ParmDefinition = '@OutString varchar(max) OUTPUT' 
--格式化sql 
set @sql = 'Select @OutString = '  
+ Substring(@sql,1 , len(@sql) -1) +  
' From dbo.Forms i  ' --需要根据目标模式进行更新 
+ ' inner join #tempTrigT d on 
i.id = d.id'  --需要根据目标模式进行更新 
--执行sql并在输出参数中检索所需的列列表 
exec sp_executesql @sql, @ParmDefinition, @OutString OUT
DECLARE @Items VARCHAR(max)
 set @Items = @OutString;
 DECLARE @Item VARCHAR(50)
 DECLARE @Pos INT
 DECLARE @Loop BIT
 SELECT @Loop = CASE WHEN LEN(@OutString) > 0 THEN 1 ELSE 0 END
 WHILE (SELECT @Loop) = 1
 BEGIN
 SELECT @Pos = CHARINDEX(',', @OutString, 1)
 IF @Pos > 0
 BEGIN
 SELECT @Item = SUBSTRING(@OutString, 1, @Pos - 1)
 SELECT @OutString = SUBSTRING(@OutString, @Pos + 1, LEN(@OutString) - @Pos)
----------------------------------
set @audit_field = @Item;
set @sql = 'select @audit_oldvalue=[' +@Item +'] from #tempTrigT'; --错误行
EXEC SP_EXECUTESQL @sql,N'@audit_oldvalue sql_variant OUTPUT',@audit_oldvalue OUTPUT -- 如果插入，则@audit_oldvalue获取新值
set @sql = 'select @audit_value=i.[' +@Item +'] from dbo.Forms i  inner join #tempTrigT d on i.id = d.id';
EXEC SP_EXECUTESQL @sql,N'@audit_value sql_variant OUTPUT',@audit_value OUTPUT
if @Action = 'U'
begin
    insert into [dbo].[AuditTrailForms]([TSid],[TSField],[OldValue],[NewValue],[changedate],[Change_Action],[Change_user],[Columns_Updated])
    select id,@audit_field, @audit_oldvalue, @audit_value,getdate(),@Action, coalesce(ModifiedBy,suser_name()), @Items
    from inserted 
end
     END
 ELSE
 BEGIN
 SELECT @Item = @OutString
 SELECT @Loop = 0
 END
 END