SQL触发器：更改nvarchar大小

Question

我有几个列nvarchar（max）的表。我正在构建一个触发器，试图将单元格存储在sql_Variant中。 Nvarchar（max）不能存储在sql_variant中。如何检查它是否为nvarchar（max）然后将其转换为nvarchar（4000）以便我可以避免这种情况？

编辑：最有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但#tempTrigT中的['+ @ Item +']可以是nvarchar（max）

如果我在输入表中编辑某些内容，则会出现此错误：

  

操作数类型clash nvarchar（max）与sql_variant

不兼容

表格结构：

输入表

输出表

我要转换的代码：

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'
--Define output parameter 
set @ParmDefinition = '@OutString varchar(max) OUTPUT' 
--Format sql 
set @sql = 'Select @OutString = '  
+ Substring(@sql,1 , len(@sql) -1) +  
' From dbo.Forms i  ' --Will need to be updated for target schema 
+ ' inner join #tempTrigT d on 
i.id = d.id'  --Will need to be updated for target schema 
--Execute sql and retrieve desired column list in output parameter 
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'; --ERROR LINE
EXEC SP_EXECUTESQL @sql,N'@audit_oldvalue sql_variant OUTPUT',@audit_oldvalue OUTPUT -- If inserted @audit_oldvalue gets the new value

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

Answer 1

您应该能够将其转换为VARCHAR（4000）。

Select @sql = @sql + 'Case when IsNull(CAST(i.[' + Column_Name +  
'] AS VARCHAR(4000)),0) = IsNull(CAST(d.[' + Column_name + '] AS VARCHAR(4000)),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'

或者，如果您只想对这些列执行此操作（除非出于性能原因，我认为不会出现问题）。

Select @sql = @sql + 'Case when IsNull(' 
    + CASE WHEN CHARACTER_MAXIMUM_LENGTH = -1 THEN 'CAST(' ELSE '' END 
    + 'i.[' + Column_Name +  ']' 
    + CASE WHEN CHARACTER_MAXIMUM_LENGTH = -1 THEN ' AS VARCHAR(4000))' ELSE '' END + 
    ',0) = IsNull('
    + CASE WHEN CHARACTER_MAXIMUM_LENGTH = -1 THEN 'CAST(' ELSE '' END 
    + 'd.[' + Column_name + ']'
    + CASE WHEN CHARACTER_MAXIMUM_LENGTH = -1 THEN ' AS VARCHAR(4000))' ELSE '' END + 
    + ',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'