SQL Fallback Row?

Question

I'm using MySQL 5.1 with PHP and I was wondering if it would be possible to run a query that will select a specific row using the primary key, but if it doesn't exist, to return a different one.

For example: select * from table1 where id="a" else id="b"

Answer 1

select *
  from table1 
  where id="a"
union all
select * 
  from table1
  where id="b"
  and no exists (
    select * 
      from table1
      where id="a");