您的位置：鳄鱼CMS > jpa一对多查询 JPA 查询IN一对多示例

jpa一对多查询 JPA 查询IN一对多示例

2023-04-18 19:31 Java教程

jpa一对多查询

JPA一对多查询是指在一个实体中有一个属性是另一个实体的集合，这种关系被称为一对多关系。JPA提供了一些API来帮助我们实现这种关系，以便我们可以使用JPQL语句来查询数据库中的数据。

要实现JPA一对多查询，首先需要在实体之间建立关联。在JPA中，有两种方式可以建立实体之间的关联：@OneToMany注释和@ManyToOne注释。@OneToMany注释用于表明一个实体有多个相关的实体，而@ManyToOne注释则表明多个实体都有相同的相关实体。

// 定义Department 实体 
@Entity 
public class Department { 

    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    private Long id; 

    private String name; 

    // 声明Employee 和 Department 的一对多关联 
    @OneToMany(mappedBy = "department") 
    private List<Employee> employees; 

    // getter and setter ...  
}  

 // 定义 Employee 实体  
@Entity  
public class Employee {  

    @Id  
    @GeneratedValue(strategy = GenerationType.IDENTITY)  
    private Long id;  

    private String name;  

     // 声明Employee 和 Department 的多对一关联  
     @ManyToOne  
     private Department department;  

     // getter and setter ...  
}

当我们已经建立了实体之间的关联后，就可以使用JPQL语句来执行JPA一对多查询了。例如，如果要查询所有部门中员工的姓名，可以使用如下JPQL语句：

SELECT e.name FROM Department d JOIN d.employees e

JPA 查询IN一对多示例

JPA教程 - JPA查询IN一对多示例

以下JPQL显示如何在一对多映射中使用IN运算符。

executeAndPrintQuery("SELECT DISTINCT p FROM Employee e, IN(e.directs) p");

例子

下面的代码来自Department.java。

package cn..common;


import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {
    @Id
    private int id;
    private String name;
    @OneToMany(mappedBy="department")
    private Collection<Employee> employees;

    public Department() {
        employees = new ArrayList<Employee>();
    }
    
    public void setId(int id) {
      this.id = id;
    }

    public void setName(String name) {
      this.name = name;
    }

    public void setEmployees(Collection<Employee> employees) {
      this.employees = employees;
    }

    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public Collection<Employee> getEmployees() {
        return employees;
    }

    public String toString() {
        return "Department no: " + getId() + 
               ", name: " + getName();
    }
}

以下代码来自Project.java。

package cn..common;


import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany(mappedBy="projects")
    private Collection<Employee> employees;

    public Project() {
        employees = new ArrayList<Employee>();
    }

    public void setId(int id) {
      this.id = id;
    }

    public void setName(String name) {
      this.name = name;
    }

    public void setEmployees(Collection<Employee> employees) {
      this.employees = employees;
    }

    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public Collection<Employee> getEmployees() {
        return employees;
    }
    
    public String toString() {
        return "Project id: " + getId() + ", name: " + getName();
    }
}

以下代码来自Employee.java。

package cn..common;



import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.OneToMany;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entity
public class Employee {
    @Id
    private int id;
    private String name;
    private long salary;
    @Temporal(TemporalType.DATE)
    private Date startDate;
    
    @ManyToOne
    private Employee manager;
    
    @OneToMany(mappedBy="manager")
    private Collection<Employee> directs;

    @ManyToOne
    private Department department;
    
    @ManyToMany 
    private Collection<Project> projects;

    public Employee() {
        projects = new ArrayList<Project>();
        directs = new ArrayList<Employee>();
    }

    public void setId(int id) {
      this.id = id;
    }

    public void setName(String name) {
      this.name = name;
    }

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

    public void setStartDate(Date startDate) {
      this.startDate = startDate;
    }

    public void setManager(Employee manager) {
      this.manager = manager;
    }

    public void setDirects(Collection<Employee> directs) {
      this.directs = directs;
    }

    public void setDepartment(Department department) {
      this.department = department;
    }

    public void setProjects(Collection<Project> projects) {
      this.projects = projects;
    }

    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }

    public long getSalary() {
        return salary;
    }

    public Date getStartDate() {
        return startDate;
    }
    
    public Department getDepartment() {
        return department;
    }
    
    public Collection<Employee> getDirects() {
        return directs;
    }
    
    public Employee getManager() {
        return manager;
    }

    public Collection<Project> getProjects() {
        return projects;
    }
    
    public String toString() {
        return "Employee " + getId() + 
               ": name: " + getName() +
               ", salary: " + getSalary() +
               ", dept: " + ((getDepartment() == null) ? null : getDepartment().getName());
    }
}

下面的代码来自PersonDaoImpl.java。

package cn..common;

import java.util.Arrays;
import java.util.Date;
import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test() {
    Employee emp = new Employee();
    emp.setName("Tom");
    emp.setSalary(123);
    emp.setStartDate(new Date());
    emp.setId(1);

    Project pro = new Project();
    pro.setName("Design");
    pro.getEmployees().add(emp);

    Department dept = new Department();
    dept.setName("Dept");
    dept.getEmployees().add(emp);

    emp.setDepartment(dept);
    emp.getProjects().add(pro);

    em.persist(dept);
    em.persist(pro);
    em.persist(emp);

    executeAndPrintQuery("SELECT DISTINCT p FROM Employee e, IN(e.directs) p");
  }
  private void executeAndPrintQuery(String queryString) {
    try {
        Query query = em.createQuery(queryString);
        printQueryResult(queryString, query.getResultList());
    } finally {
        em.close();
    }
}
  private void printQueryResult(String queryString, List result) {
    System.out.println("EJB QL: " + queryString+"n");
    System.out.println("Result:n");
    if (result.isEmpty()) {
        System.out.println("No results Foundn");
    } else {
        for (Object o : result) {
            System.out.println(resultAsString(o));
        }
    }
  }
  private String resultAsString(Object o) {
    if (o instanceof Object[]) {
        return Arrays.asList((Object[])o).toString();
    } else {
        return String.valueOf(o);
    }
}
  @PersistenceContext
  private EntityManager em;
}

下载 Query_In_OneToMany.zip

上面的代码生成以下结果。

以下是数据库转储。

Table Name: DEPARTMENT
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 0

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Dept





Table Name: EMPLOYEE
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Tom

    Column Name: SALARY,
    Column Type: BIGINT:
    Column Value: 123

    Column Name: STARTDATE,
    Column Type: DATE:
    Column Value: 2014-12-29

    Column Name: DEPARTMENT_ID,
    Column Type: INTEGER:
    Column Value: 0

    Column Name: MANAGER_ID,
    Column Type: INTEGER:
    Column Value: null





Table Name: EMPLOYEE_PROJECT
 Row:
    Column Name: EMPLOYEES_ID,
    Column Type: INTEGER:
    Column Value: 1

    Column Name: PROJECTS_ID,
    Column Type: INTEGER:
    Column Value: 0





Table Name: PROJECT
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 0

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Design

阅读全文

以上是鳄鱼CMS为你收集整理的jpa一对多查询 JPA 查询IN一对多示例全部内容。

声明：本站所有文章资源内容，如无特殊说明或标注，均为采集网络资源。如若本站内容侵犯了原著者的合法权益，可联系本站删除。

java的格式 Java 格式介绍

2023-06-11 Java教程

...
java中打印字符串 Java 打印格式字符串

2023-03-17 Java教程

Java格式 -Java 打印格式字符串使用Formatter格式化数据需要两种类型的输入:格式字符串值列表格式字符串定义如何格式化值列表。...
java正则表达式模式有哪些 Java 正则表达式模式

2023-05-06 Java教程

Java正则表达式教程 -Java正则表达式模式包 java.util.regex 包含三个类，以支持正则表达式的完整版本。模式匹配PatternSyntaxEx...
java 日期格式 Java 日期格式模式

2023-03-11 Java教程

Java格式 -Java日期格式模式SimpleDateFormat日期和时间格式由日期和时间模式字符串指定。在格式字符串中，从“A到“Z和从“a到...
java当前线程sleep Java 当前线程

2023-03-06 Java教程

Java线程教程 -Java当前线程一个语句可以由不同的线程在不同的时间执行。Thread类静态方法currentThread()返回调用此方法的Threa...