連接表

描述

在數據庫中存儲類層次結構的另一種策略稱為 Joined Table。它有一個特殊的註釋：

@Inheritance(strategy = InheritanceType.JOINED)

我們班級的一個例子：

@Inheritance(strategy = InheritanceType.JOINED)
@Entity
class User {
  int id;
  String name;
  LocalDate birthday;
}

@Entity
class Employee extends User {
 	String occupation;
 	int salary;
 	LocalDate join;
}

@Entity
class Client extends User {
   String address;
}

當使用這個註解時，Hibernate 將期望在數據庫中為每個類及其子類創建一個單獨的表。從它們中選擇數據時，您將不得不使用 SQL JOIN 運算符。

數據庫架構示例：

CREATE TABLE user {
  id INT,
  name VARCHAR,
  birthday DATE
}

CREATE TABLE employee {
  id INT,
  occupation VARCHAR,
  salary INT,
  join DATE
}

CREATE TABLE client {
  id INT,
  address VARCHAR
}

如果您決定從表中獲取某些客戶端的數據，那麼 Hibernate 將不得不使用 JOIN 來連接表：

SELECT u.id, u.name, u.birthday, c.address FROM user u JOIN client c ON u.id = c.id;

@PrimaryKeyJoinColumn

子實體類在表中有一列引用父實體類的對象 ID。此列的名稱默認等於父類列的名稱。

例子：

@Inheritance(strategy = InheritanceType.JOINED)
@Entity
class User {
  @Id
  int user_identifier;
  String name;
  LocalDate birthday;
}

@Entity
class Employee extends User {
 	String occupation;
 	int salary;
 	LocalDate join;
}

@Entity
class Client extends User {
   String address;
}

然後數據庫表將如下所示：

CREATE TABLE user {
  user_identifier INT,
  name VARCHAR,
  birthday DATE
}

CREATE TABLE employee {
  user_identifier INT,
  occupation VARCHAR,
  salary INT,
  join DATE
}

CREATE TABLE client {
  user_identifier INT,
  address VARCHAR
}

如果要覆蓋依賴表中的列名，則需要使用@PrimaryKeyJoinColumn註解 。例子：

@Inheritance(strategy = InheritanceType.JOINED)
@Entity
class User {
  @Id
  int user_identifier;
  String name;
  LocalDate birthday;
}

@Entity
@PrimaryKeyJoinColumn(name=”user_id”)
class Employee extends User {
 	String occupation;
 	int salary;
 	LocalDate join;
}

@Entity
@PrimaryKeyJoinColumn(name=”user_id2”)
class Client extends User {
   String address;
}

然後數據庫表將如下所示：

CREATE TABLE user {
  user_identifier INT,
  name VARCHAR,
  birthday DATE
}

CREATE TABLE employee {
  user_id INT,
  occupation VARCHAR,
  salary INT,
  join DATE
}

CREATE TABLE client {
  user_id2 INT,
  address VARCHAR
}