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