批量修改 表结构

declare @index int;
declare @sql nvarchar(2000);
set @index=1;
while @index<=851

begin

set @sql='alter table visitlog'+CONVERT(NVARCHAR(20), @index)+' alter column [OS] nvarchar(30) null;';
set @index=@index+1;
begin try
exec(@sql);
end try
BEGIN CATCH
    print 'error';    print @index;
    print ERROR_MESSAGE();
end catch


end