JPA 查询Distinct多对一示例

JPA教程 - JPA查询Distinct多对一示例

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。

例子

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;

@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.w3cschool.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();
    }
}

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;

@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 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;
@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());
    }
}

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