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one to many vs many to many - Books and authors

The bidirectional associations allow you to navigate both sides of this relationship both in Object space as well as in JPA/Hibernate Query Language.

In your example, a database many-to-many association is mandatory (meaning you have a BOOK, an AUTHOR and a BOOK_AUTHOR link table). Hibernate offers two possibilities for mapping this relationship:

  1. You can model this with two entities: Book and Author, each one having a @ManyToMany association to the other (one being the owning side while the other being the inverse one)

  2. You can have three entities: Book, Author and BookAuthor and this time Book have a @OneToMany association to BookAuthor and Author has also a @OneToMany association to BookAuthor. This option allows you two map additional link table columns (BOOK_AUTHOR creation_time).

Categories : Database

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Compute the total number of accepted papers that are authored by at least two authors
You could just wrap the existing query in another aggregate query: SELECT COUNT(*) FROM (SELECT COUNT(AuID) FROM paperauthorid GROUP BY PPNum HAVING COUNT(AuID)>= 2) t

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JSON Google Books and Ajax
You would need to decode it in client side to pass the array to server side. Something like : $.ajax({ type: 'POST', url: 'addIsbnScript.php', data: { 'isbn' : isbn, 'title' : entry.volumeInfo.title, 'subtitle' : entry.volumeInfo.subtitle, 'authors' : JSON.stringify(entry.volumeInfo.authors), 'description' : entry.volumeInfo.description

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Efficient query to choose books not previously issued
RANDOM() is very slow. Try adding a "random_order" column to your tables and assigning a random number to each row with the RANDOM() function. Make sure those columns are indexed, and then after that you can use a query to select the maximum random_order item. Since the random column is random, the maximum is not meaningful except to "take one book off the top" of the list of recommendations. This

Categories : Mysql
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