Which of the following SQL statements is not valid? a. SELECT isbn FROM books MINUS SELECT isbn FROM orderitems; b. SELECT isbn, name FROM books, publisher WHERE books.pubid (+) ¼ publisher.pubid (+);...

Which of the following SQL statements is not valid?

a. SELECT isbn

FROM books

MINUS

SELECT isbn

FROM orderitems;

b. SELECT isbn, name

FROM books, publisher

WHERE books.pubid (+) ¼ publisher.pubid (+);

c. SELECT title, name

FROM books NATURAL JOIN publisher

d. All the above SQL statements are valid.