rel-ops.txt

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(
	select Book.title, Book.price
	from   Book, PUBLISHER
	where  Book.PublisherID = PUBLISHER.PublisherID and PUBLISHER.PublisherName = 'MIT Press'
)
union
(
	select Book.title, Book.price
	from   Book, PUBLISHER
	where  Book.PublisherID = PUBLISHER.PublisherID and PUBLISHER.PublisherName = 'University of Chicago Press'
);

use select adn join operations.

In order to search books published by MIT Press and University of Chicago Press, then print them in title and price.

Union operation can join this two select together.