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The a in the a.* is an alias. He used the 'a' bolded below to give the name to the select statement in front of that point so he could refrence it in the beginning. The a.* portion is refrencing all of the rows and columns in the 'a' query.
select city,length_city from
(select a., rownum r from (select length(city) length_city,city from station order by length_city, city) a) where r in (1,(select count() from station));