You can use subqueries that are introduced with exists and not exists for two set theory operations: intersection and difference. The intersection of two sets contains all elements that belong to both of the original sets. The difference contains the elements that belong only to the first set.
The intersection of authors and publishers over the city column is the set of cities in which both an author and a publisher are located:
select distinct city from authors where exists (select * from publishers where authors.city = publishers.city)
city -------------------- Berkeley
The difference between authors and publishers over the city column is the set of cities where an author lives but no publisher is located, that is, all the cities except Berkeley:
select distinct city from authors where not exists (select * from publishers where authors.city = publishers.city)
city -------------------- Gary Covelo Oakland Lawrence San Jose Ann Arbor Corvallis Nashville Palo Alto Rockville Vacaville Menlo Park Walnut Creek San Francisco Salt Lake City
