继承映射（二）每个类一张表JOINED

Company,EmployeeDAO都不变。

Employee变为：

package com.persia.joined;

import java.io.Serializable;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;

@Entity
@Table(name="employee")
@Inheritance(strategy=InheritanceType.JOINED)
public class Employee implements Serializable {

	private Integer id;
	private String name;
	private Company company;
	
	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	@ManyToOne(cascade=CascadeType.PERSIST,optional=false)
	@JoinColumn(name="company_id")
	public Company getCompany() {
		return company;
	}
	public void setCompany(Company company) {
		this.company = company;
	}
	
	
}

两个继承类：

HourEmployee:

package com.persia.joined;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;

@Entity
@Table(name="HourEmployee")
@PrimaryKeyJoinColumn(name="hid")//把主键对应的列名改为"hid"
public class HourEmployee extends Employee {

	private Float rate;

	@Column(nullable=true)
	public Float getRate() {
		return rate;
	}

	public void setRate(Float rate) {
		this.rate = rate;
	}
	
	
}

SalaryEmployee:

package com.persia.joined;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
import javax.persistence.PrimaryKeyJoinColumn;

@Entity
@Table(name="SalaryEmployee")
@PrimaryKeyJoinColumn(name="sid")
public class SalaryEmployee extends Employee {

	private Float salary;

	@Column(nullable=true)
	public Float getSalary() {
		return salary;
	}

	public void setSalary(Float salary) {
		this.salary = salary;
	}
	
	
}

--------------------------------------------------------------------------------------------------------------------------

生成的数据库表：

mysql> desc company;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set (0.06 sec)

mysql> desc employee;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| name       | varchar(255) | YES  |     | NULL    |                |
| company_id | int(11)      | NO   | MUL | NULL    |                |
+------------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> desc houremployee;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| hid   | int(11) | NO   | PRI | NULL    |       |
| rate  | float   | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> desc salaryemployee;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| sid    | int(11) | NO   | PRI | NULL    |       |
| salary | float   | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

----------------------------------------------------------------------------------------------------------------------------------

插入数据：

mysql> select * from company;
+----+----------+
| id | name     |
+----+----------+
|  1 | waltmart |
+----+----------+
1 row in set (0.00 sec)

mysql> select * from employee;
+----+----------+------------+
| id | name     | company_id |
+----+----------+------------+
|  1 | salary-a |          1 |
|  2 | hour-b   |          1 |
+----+----------+------------+
2 rows in set (0.00 sec)

mysql> select * from houremployee;
+-----+------+
| hid | rate |
+-----+------+
|   2 |  888 |
+-----+------+
1 row in set (0.00 sec)

mysql> select * from salaryemployee;
+-----+--------+
| sid | salary |
+-----+--------+
|   1 |   9999 |
+-----+--------+
1 row in set (0.00 sec)

--------------------------------------------------------------------------------------------------------------------------------------------

修改HourEmployee的数据，就houremployee表的数据改变：

mysql> select * from houremployee;
+-----+------+
| hid | rate |
+-----+------+
|   2 |   95 |
+-----+------+
1 row in set (0.00 sec)

-------------------------------------------------------------------------------------------------------------------------------------

删除SalaryEmployee:

mysql> select * from company;
+----+----------+
| id | name     |
+----+----------+
|  1 | waltmart |
+----+----------+
1 row in set (0.00 sec)

mysql> select * from employee;
+----+--------+------------+
| id | name   | company_id |
+----+--------+------------+
|  2 | hour-b |          1 |
+----+--------+------------+
1 row in set (0.00 sec)

mysql> select * from houremployee;
+-----+------+
| hid | rate |
+-----+------+
|   2 |   95 |
+-----+------+
1 row in set (0.00 sec)

mysql> select * from salaryemployee;
Empty set (0.00 sec)

 

删除后父类跟子类的表都删除了相关记录。

--------------------------------------------------------------------------------------------------------------------------

删除Company:

mysql> select * from salaryemployee;
Empty set (0.00 sec)

mysql> select * from company;
Empty set (0.00 sec)

mysql> select * from employee;
Empty set (0.00 sec)

mysql> select * from salaryemployee;
Empty set (0.00 sec)

mysql> select * from houremployee;
Empty set (0.00 sec)

 

全部级联删除。

 

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优点：

１.支持多态关联和查询

２.允许子类的属性定义not null

缺点：

1.查询性能不如SINGLE_TABLE

2.当对查询性能要求不高，子类属性非常多，而且需要对其子类属性定义not null 用此策略。