如何使用 sql 查询休眠多对多列

Question

我正在使用 java hibernate 来存储我的数据实体。我想知道使用 postgresql-psql 命令在 @ManyToMany 列中选择数据的 sql 命令。

对于普通列，我可以运行：

SELECT id FROM university;

但现在我有以下大学实体：

@Entity
@Table(name = "university")
public class University {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "ID")
    private Long id;

    @ManyToMany(fetch=FetchType.EAGER)
    @JoinColumn(name="students" /* referencedColumnName="id" */)
    private List<Student> students;
}
@Entity
@Table(name = "student", uniqueConstraints = { @UniqueConstraint(columnNames={"name"})})
public class Student
{
    @Id
    private Long id;

    @NotBlank
    @NotNull
    private String       name;
}

问题是，我不知道学生列表在psql中叫什么。

当我跑步时：

SELECT students FROM university;

我明白了：

ERROR:  column "students" does not exist

当我输入时：

\d university

我得到（不是实际数据：对学生/大学示例匿名的数据）：

      Column      |            Type             | Collation | Nullable | Default 
------------------+-----------------------------+-----------+----------+---------
 id               | bigint                      |           | not null | 
Indexes:
    "university_pkey" PRIMARY KEY, btree (id)
    "uk_rwpd2frv6wtkgqtxn3envk3i8" UNIQUE CONSTRAINT, btree (name)
Referenced by:
    TABLE "university_students" CONSTRAINT "fkdkjk4jgutu64g937gkknybax2" FOREIGN KEY (university) REFERENCES university(id)

Answer 1

你有一个表'university_students'，你能做'select * from university_students'吗

我认为你有这样的结构：

学生：

id	name
0	first_student

大学：

id
3

大学学生：

university_id	students_id
3	0

所以你唯一需要做的就是： SELECT * FROM student WHERE id IN (SELECT students_id FROM university_students WHERE university_id = 3)

这将在大学学生表中搜索大学ID等于3的所有学生ID，所有将与表学生匹配。

如果您只想将他们的名字替换为 * 为 name，如下所示： SELECT name FROM student WHERE id IN (SELECT students_id FROM university_students WHERE university_id = 3)

Answer 2

好的得到了我想要的，灵感来自：https://stackoverflow.com/a/3486662/2396744

鉴于：

大学：

database=> SELECT id FROM university;
 id 
----
  2
  3
  4
  5
(4 rows)

学生：

database=> SELECT id,name FROM students;
 id | name       
----+----------
  4 | Jack
  3 | Jill
  2 | Jonas
(3 rows)

大学学生：

database=> SELECT * FROM university_students;
 university_id | students_id 
---------------+---------
             3 |       3
             3 |       2
             4 |       4
             4 |       2
             5 |       3
             5 |       2
(6 rows)

查询变为：

database=> SELECT u.id,us.university_id,us.students_id,s.name
FROM students as s, university as u, university_students as us
WHERE u.id = 5
AND us.university_id = u.id
AND us.student_id = s.id;
 id | university_id | student_id | name     
----+---------------+------------+----------
  5 |             5 |          3 | Jill
  5 |             5 |          2 | Jonas
(2 rows)

Answer 3

你可以并且你真的应该学习使用java spring boot的JpaRepository，它易于使用，你可以用它做很多事情，例如我有这个类：Client.java

@Entity
public class Client {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;
    @Column(unique = true, nullable = false) private String email, username;
    @Column(unique = true) private String phone;
    @Column(nullable = false) private String password, firstName, lastName;
    private boolean active;
    private long timestampCreation;
    private double coefficientReducingBuyer;
    private long score;
    // User privileges, roles, Address
    @ManyToMany(mappedBy = "clients", fetch = FetchType.EAGER) private Set<Privilege> privileges;
    @ManyToOne(fetch = FetchType.EAGER) private Role role;
     @OneToMany(mappedBy = "client", fetch = FetchType.EAGER, cascade = CascadeType.ALL) private Set<AddressClient> addressClients;
    // Market
    @OneToMany(fetch = FetchType.EAGER) private Set<Order> orders;
    @OneToOne(mappedBy = "client", cascade = CascadeType.ALL) private ShoppingCart shopping;
    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL) private Set<Favorite> favorites;
}

public interface ClientRepository extends JpaRepository<Client, Integer> {
    // Find all client by phone number
    Client findByPhone(String phone);
    // Find all client by phone number which is containing some element of my string
    List<Client> findByPhoneIsContaining(String phone);
    // Same but ignoring capital letter a and A is the same here
    List<Client> findByPhoneIgnoreCaseIsContaining(String phone);
    // Find by other table
    List<Client> findByRole(Role role);
    // Find by other table with a specific value in it
    Set<Client> findByPrivileges_Name(String privilege);
}

您不需要添加@Table，仅当您想更改表的名称时。

这里我的表将命名为client，但是当我这样做时我可以将名称更改为user：@Table(name="user")，所以当你不需要它时不要使用@Table。