question about EJB QL

[vitesse wei]

Question:

A Reader entity has a one-to-many, bidirectional relationship with a Book entity. Two Reader entities are persisted, each having two Book entities associated with them. For example, reader 1 has book a and book b, while reader 2 has book c and book d. Which query returns a Collection of
fewer than four elements?
A. SELECT b.reader FROM Book b
B. SELECT r FROM Book b INNER JOIN b.reader r
C. SELECT r FROM Reader r INNER JOIN r.books b
D. SELECT r from Book b LEFT JOIN b.reader r LEFT JOIN FETCH r.books

Given answer is C,why A,B is wrong?

[Meherdad Bomanbehram]

Hi,
The Reader entity has 2 entries whereas the book entity has 4 entries.
we want a collection thats less than 4 .
option A will give all the readers from the book entity thats 4 entries
option B will give all the reader entities joined to the book entity thats also 4 .
option C will give all the readers from the reader entity thjat have books . the result set is a collection of 2 hence this is the correct answer.