SQL get last rows in table WITHOUT primary ID

If your table has no primary key or your primary key is not orderly... you can try the code below... if you want see more last record, you can change the number in code

Select top (select COUNT(*) from table) * From table
EXCEPT
Select top ((select COUNT(*) from table)-(1)) * From table

If you need to select 1 column from a table of 800,000 rows where that column is the min or max possible value, and that column is not indexed, then the unassailable fact is that SQL will have to read every row in the table in order to identify that min or max value.

(An aside, on the face of it reading all the rows of an 800,000 row table shouldn't take all that long. How wide is the column? How often is the query run? Are there concurrency, locking, blocking, or deadlocking issues? These may be pain points that could be addressed. End of aside.)

There are any number of work-arounds (indexes, views, indexed views, peridocially indexed copies of the talbe, run once store result use for T period of time before refreshing, etc.), but virtually all of them require making permanent modifications to the database. It sounds like you are not permitted to do this, and I don't think there's much you can do here without some such permanent change--and call it improvement, when you discuss it with your project manager--to the database.

I assume that when you are saying 'last rows', you mean 'last created rows'.

Even if you had primary key, it would still be not the best option to use it do determine rows creation order. There is no guarantee that that the row with the bigger primary key value was created after the row with a smaller primary key value.
Even if primary key is on identity column, you can still always override identity values on insert by using set identity_insert on.

It is a better idea to have timestamp column, for example CreatedDateTime with a default constraint. You would have index on this field.
Then your query would be simple, efficient and correct:

select top 1 *
from MyTable
order by CreatedDateTime desc

If you don't have timestamp column, you can't determine 'last rows'.