嵌套查询-NOT IN（从用户u中选择ub.book_id，在User_Book ub中选择u.id = 5，而u.id = ub.user_id）
我尝试在没有嵌套查询的情况下运行查询，这确实可行，但是当我添加嵌套查询时，得到的是空集。
总代码：
public Collection<Bookmark> getBooks(boolean isBookmarked, long userId) {
Collection<Bookmark> result = new ArrayList<>();
try {
Class.forName("com.mysql.cj.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
//Dao connection
try (Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test?useSSL=false", "root",
"password"); /* a) Database User Profile: root is who the user is b) Database user password */
Statement stmt = conn.createStatement();) /* execute mysql queries */ {
String query = "";
if (!isBookmarked) {
query = "Select b.id, title, image_url, publication_year, GROUP_CONCAT(a.name SEPARATOR ',') AS authors, book_genre_id, " +
"amazon_rating from Book b, Author a, Book_Author ba where b.id = ba.book_id and ba.author_id = a.id and " +
"b.id NOT IN (select ub.book_id from User u, User_Book ub where u.id = " + userId +
" and u.id = ub.user_id) group by b.id";
} /*else {
query = "Select b.id, title, image_url, publication_year, GROUP_CONCAT(a.name SEPARATOR ',') AS authors, book_genre_id, " +
"amazon_rating from Book b, Author a, Book_Author ba where b.id = ba.book_id and ba.author_id = a.id and " +
"b.id IN (select ub.book_id from User u, User_Book ub where u.id = " + userId +
" and u.id = ub.user_id) group by b.id";
}*/
ResultSet rs = stmt.executeQuery(query);
while (rs.next()) {
long id = rs.getLong("id");
String title = rs.getString("title");
String imageUrl = rs.getString("image_url");
int publicationYear = rs.getInt("publication_year");
// String publisher = rs.getString("name");
String[] authors = rs.getString("authors").split(",");
int genre_id = rs.getInt("book_genre_id");
BookGenreEnum genre = BookGenreEnum.values()[genre_id];
double amazonRating = rs.getDouble("amazon_rating");
System.out.println("id: " + id + ", title: " + title + ", publication year: " + publicationYear + ", authors: " + String.join(", ", authors) + ", genre: " + genre + ", amazonRating: " + amazonRating);
Bookmark bookmark = BookmarkManager.getInstance().createBook(id, title, imageUrl, publicationYear, null, authors, genre, amazonRating/*, values[7]*/);
result.add(bookmark);
}
} catch (SQLException e) {
e.printStackTrace();
}
return result;
}
嵌套查询：
Select b.id, title, image_url, publication_year, GROUP_CONCAT(a.name SEPARATOR ',') AS authors, book_genre_id, amazon_rating
from Book b, Author a, Book_Author ba
where b.id = ba.book_id and ba.author_id = a.id and b.id
NOT IN (select ub.book_id from User u, User_Book ub where u.id = 5 and u.id = ub.user_id) group by b.id;
嵌套查询-MySQL结果：
Empty set (0.00 sec)
无需嵌套查询：
Select b.id, title, image_url, publication_year, p.name, GROUP_CONCAT(a.name SEPARATOR ',') AS authors, book_genre_id, amazon_rating, created_date
from Book b, Publisher p, Author a, Book_Author ba
where b.publisher_id = p.id and b.id = ba.book_id and ba.author_id = a.id
group by b.id;
无需嵌套查询-MySQL结果
mysql> select * from book \G;
*************************** 1. row ***************************
id: 1
title: The Complete Software Developer's Career Guide
image_url: https://images-na.ssl-images-amazon.com/images/I/51yr12gkjRL._SY346_.jpg
publication_year: 2017
publisher_id: 1
book_genre_id: 10
amazon_rating: 4.7
kid_friendly_status: 0
kid_friendly_marked_by: 4
shared_by: 5
created_date: 2019-06-22 09:17:29
*************************** 2. row ***************************
id: 2
title: The Rational Male
image_url: https://images-na.ssl-images-amazon.com/images/I/51dS%2Bd%2BwZEL._SX322_BO1,204,203,200_.jpg
publication_year: 2013
publisher_id: 2
book_genre_id: 8
amazon_rating: 4.5
kid_friendly_status: 1
kid_friendly_marked_by: 3
shared_by: 5
created_date: 2019-06-22 09:17:29
*************************** 3. row ***************************
id: 3
title: The 48 Laws of Power
image_url: https://images-na.ssl-images-amazon.com/images/I/41EhEN9nsmL._SY346_.jpg
publication_year: 2000
publisher_id: 3
book_genre_id: 5
amazon_rating: 4.5
kid_friendly_status: 1
kid_friendly_marked_by: 5
shared_by: 5
created_date: 2019-06-22 09:17:29
*************************** 4. row ***************************
id: 4
title: Rich Dad Poor Dad
image_url: https://images-na.ssl-images-amazon.com/images/I/51N2wpwLcVL._SX342_.jpg
publication_year: 2017
publisher_id: 4
book_genre_id: 8
amazon_rating: 4.6
kid_friendly_status: 0
kid_friendly_marked_by: 3
shared_by: 4
created_date: 2019-06-22 09:17:30
*************************** 5. row ***************************
id: 5
title: Eat That Frog!
image_url: https://images-na.ssl-images-amazon.com/images/I/51N38zCWljL._SX342_.jpg
publication_year: 2017
publisher_id: 5
book_genre_id: 8
amazon_rating: 4.6
kid_friendly_status: 0
kid_friendly_marked_by: 4
shared_by: 5
created_date: 2019-06-22 09:17:30
这表示要显示特定用户已为其添加书签的所有图书。