有个关于sp_executesql有关问题请问

-- 0. 传入参数@tablename   NVARCHAR(32)   = '',           --表名@fieldname   NVARCHAR(32)   = '',           --字段名@sqlstring   NVARCHAR(1024) = '',           --查询条件，不含WHERE@results     NVARCHAR(256)  = ''  OUTPUT    --输出结果ASDECLARE @sql   NVARCHAR(2048);BEGIN    BEGIN TRAN        SET @sql =N'SELECT @results = ' + @fieldname + ' FROM ' + @tablename;        IF (LTRIM(RTRIM(@sqlstring))<>'' AND @sqlstring IS NOT NULL)            SET @sql = @sql + ' WHERE ' + @sqlstring;        ELSE            SET @sqlstring = '';        EXECUTE sp_executesql @sql,[email protected] NVARCHAR(256) OUT',@results OUTPUT        IF @results IS NULL OR LTRIM(RTRIM(@results)) = ''            SELECT @results = '';        ELSE            SELECT @results = LTRIM(RTRIM(@results));        IF @@ERROR <> 0        BEGIN            ROLLBACK TRAN            RETURN '';        END        ELSE        BEGIN            COMMIT TRAN            RETURN @results;        ENDEND