rel-ops.txt

ssd7的本人的答案

3.
SELECT Title, Price
FROM BOOK
WHERE PublisherID IN 
(SELECT PublisherID
FROM PUBLISHER
WHERE Name = 'Electronic Industry' OR Name = 'Nwpu'
);


Because select operation can return the special colomn of the table, so if we want the price and title from
the BOOK, we can use it. Besides, we can use it retrieve the publisherid from the PUBLISHER whose name is
'Electromic Industry' or 'Nwpu'.So with the IN operation, we can retrieve the title and price of all books published 
by either of two publishers('Electromic Industry' or 'Nwpu'). 


4.
SELECT Name
FROM PUBLISHER
WHERE PublisherId IN 
(SELECT PublisherId
FROM BOOK
WHERE Title = 'Database System'
);