我有两个表 Employee 和 Department 以下是它们的实体类
Department.java
@Entity
@Table(name = "DEPARTMENT")
public class Department {
@Id
@Column(name = "DEPARTMENT_ID")
@GeneratedValue(strategy = GenerationType.AUTO)
private Integer departmentId;
@Column(name = "DEPARTMENT_NAME")
private String departmentName;
@Column(name = "LOCATION")
private String location;
@OneToMany(cascade = CascadeType.ALL, mappedBy = "department", orphanRemoval = true)
@Fetch(FetchMode.SUBSELECT)
//@Fetch(FetchMode.JOIN)
private List<Employee> employees = new ArrayList<>();
}
Employee.java
@Entity
@Table(name = "EMPLOYEE")
public class Employee {
@Id
@SequenceGenerator(name = "emp_seq", sequenceName = "seq_employee")
@GeneratedValue(generator = "emp_seq")
@Column(name = "EMPLOYEE_ID")
private Integer employeeId;
@Column(name = "EMPLOYEE_NAME")
private String employeeName;
@ManyToOne
@JoinColumn(name = "DEPARTMENT_ID")
private Department department;
}
以下是我执行时触发的查询em.find(Department.class, 1);
-- 获取模式 = fetchmode.join
SELECT department0_.DEPARTMENT_ID AS DEPARTMENT_ID1_0_0_,
department0_.DEPARTMENT_NAME AS DEPARTMENT_NAME2_0_0_,
department0_.LOCATION AS LOCATION3_0_0_,
employees1_.DEPARTMENT_ID AS DEPARTMENT_ID3_1_1_,
employees1_.EMPLOYEE_ID AS EMPLOYEE_ID1_1_1_,
employees1_.EMPLOYEE_ID AS EMPLOYEE_ID1_1_2_,
employees1_.DEPARTMENT_ID AS DEPARTMENT_ID3_1_2_,
employees1_.EMPLOYEE_NAME AS EMPLOYEE_NAME2_1_2_
FROM DEPARTMENT department0_
LEFT OUTER JOIN EMPLOYEE employees1_
ON department0_.DEPARTMENT_ID =employees1_.DEPARTMENT_ID
WHERE department0_.DEPARTMENT_ID=?
-- 获取模式 = fetchmode.subselect
SELECT department0_.DEPARTMENT_ID AS DEPARTMENT_ID1_0_0_,
department0_.DEPARTMENT_NAME AS DEPARTMENT_NAME2_0_0_,
department0_.LOCATION AS LOCATION3_0_0_
FROM DEPARTMENT department0_
WHERE department0_.DEPARTMENT_ID=?
SELECT employees0_.DEPARTMENT_ID AS DEPARTMENT_ID3_1_0_,
employees0_.EMPLOYEE_ID AS EMPLOYEE_ID1_1_0_,
employees0_.EMPLOYEE_ID AS EMPLOYEE_ID1_1_1_,
employees0_.DEPARTMENT_ID AS DEPARTMENT_ID3_1_1_,
employees0_.EMPLOYEE_NAME AS EMPLOYEE_NAME2_1_1_
FROM EMPLOYEE employees0_
WHERE employees0_.DEPARTMENT_ID=?
我只是想知道我们应该更喜欢哪一个FetchMode.JOIN？FetchMode.SUBSELECT在哪种情况下我们应该选择哪一个？