惠州JPA如何查询Distinct多对一？_北大青鸟IT学校

Java是现在非常主流的编程语言之一，很多人想转行学习IT，首选Java语言。那么，JPA如何查询Distinct多对一？下面就让我们一起来看看惠州北大青鸟老师是怎么回答的。

DISTINCT运算符在功能上等同于同名的SQL运算符。

收集结果集后，将删除重复值，以便只返回唯一结果。

以下JPQL在多对一映射中使用DISTINCT运算符。

实例

下面的代码来自Department.java。

package cn.w3cschool.common;


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

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

@Entitypublic 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.w3cschool.common;


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

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

@Entitypublic 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();
   }
}

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

import java.io.PrintWriter;
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;

@Transactionalpublic 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 e.department FROM Employee e");
 }  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 Found\n");
   } 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;
}

下面的代码来自Employee.java。

package cn.w3cschool.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;
@Entitypublic 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());
   }
}

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

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