how to select Bottom( N) rows in a table through single query.

Use TOP, with the reverse order.

SELECT TOP (30) *
FROM Table
ORDER BY LastName DESC, FirstName DESC;

Overall I agree with Rob Farley, but in case you want to preserve original order, you might want to use the following version:

SELECT *
FROM
(
     SELECT TOP (30) *
     FROM YourTable
     ORDER BY LastName DESC, FirstName Desc
)
ORDER BY LastName ASC, FirstName Asc

or if you don't like subqueries:

SELECT TOP (30)
  *,
  ROW_NUMBER() OVER (ORDER BY LastName DESC, FirstName DESC) as RowNum
FROM YourTable
ORDER BY RowNum DESC

The subquery shown above works correctly. However, in SQL 2012 and maybe other versions, you need to add an Alias name to the subquery to make the syntax correct. An " A" after the ) works.

The Row-Number example didn't work for me, the row numbers were inversely ordered, but it returned the first block, not the last block. I think a subquery is required.