This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The correct answer is : All Customers who have at least one Order.
Because the query is translated into "For Sun Java Application Server and MySQL, select c from Customer c, IN(c.orders) o actually gets translated to : SELECT DISTINCT t0.ID, t0.LASTNAME FROM CUSTOMER t0, ORDER t1 WHERE (t1.CUSTOMER_ID = t0.ID) "
But in EJB in action, the book says one can perform theta join using ',' in the "from" clause.