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jpa只查询指定字段 JPA 查询选择两个属性示例

2023-03-18 06:31 Java教程

jpa只查询指定字段

JPA是Java Persistence API的简称，它是一种用于持久化Java对象的标准技术。它可以帮助开发人员将Java对象映射到关系数据库中，并从数据库中读取和保存Java对象。JPA允许开发人员使用JPQL（Java Persistence Query Language）来执行查询，这是一种面向对象的SQL语法。

在使用JPA时，有时我们只想要查询特定字段的数据。这时候就需要使用JPA的select子句来实现。select子句允许我们在JPQL中选择要返回的字段。例如：

SELECT u.name, u.age FROM User u

上面的JPQL将返回User表中name和age字段的数据。如果我们想要返回多个表中的字段，也可以使用select子句来实现：

SELECT u.name, a.address FROM User u, Address a WHERE u.id = a.user_id

上面的JPQL将返回User表中name字段和Address表中address字段的数据。此外，我们也可以使用select子句来选择要返回的列数量：

SELECT COUNT(u) FROM User u

上面的JPQL将返回User表中总行数。总之，使用select子句能够帮助我们快速、方便地实现JPA只查询特定字段的目标。

JPA 查询选择两个属性示例

JPA教程 - JPA查询选择两个属性示例

从别名开始，我们可以使用点（.）运算符浏览实体关系。

以下JPQL从一个实体中选择两个属性。

    List l = em.createQuery("SELECT e.name, e.salary FROM Professor e")

例子

下面的代码来自PersonDaoImpl.java。

package cn..common;

import java.util.List;

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

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test() {
    prepareData();
    List l = em.createQuery("SELECT e.name, e.salary FROM Professor e")
        .getResultList();
    for (Object p : l) {
      printResult(p);
    }
  }

  private void prepareData() {
    Professor p = new Professor();
    p.setId(0);
    p.setName("TOM");

    Department d = new Department();
    d.setId(1);
    d.setName("Design");

    p.setDepartment(d);
    d.getProfessors().add(p);

    em.persist(p);
    em.persist(d);
  }

  private static void printResult(Object result) {
    if (result == null) {
      System.out.print("NULL");
    } else if (result instanceof Object[]) {
      Object[] row = (Object[]) result;
      System.out.print("[");
      for (int i = 0; i < row.length; i++) {
        printResult(row[i]);
      }
      System.out.print("]");
    } else if (result instanceof Long || result instanceof Double
        || result instanceof String) {
      System.out.print(result.getClass().getName() + ": " + result);
    } else {
      System.out.print(result);
    }
    System.out.println();
  }

  @PersistenceContext
  private EntityManager em;
}

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.Inheritance;
import javax.persistence.ManyToMany;

@Entity
@Inheritance
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany
    protected Collection<Professor> employees = new ArrayList<Professor>();
    
    public int getId() {
        return id;
    }
    
    public void setId(int projectNo) {
        this.id = projectNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String projectName) {
        this.name = projectName;
    }
    
    public Collection<Professor> getProfessors() {
        return employees;
    }
    
    public void addProfessor(Professor employee) {
        if (!getProfessors().contains(employee)) {
            getProfessors().add(employee);
        }
        if (!employee.getProjects().contains(this)) {
            employee.getProjects().add(this);
        }
    }
    
    public String toString() {
        return getClass().getName().substring(getClass().getName().lastIndexOf(".")+1) + 
                " no: " + getId() + 
                ", name: " + getName();
    }
}

以下代码来自Phone.java。

package cn..common;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Phone {
    @Id
    private long id;
    private String number;
    private String type;
    @ManyToOne
    Professor employee;
    
    public long getId() {
        return id;
    }
    
    public void setId(long id) {
        this.id = id;
    }
    
    public String getNumber() {
        return number;
    }
    
    public void setNumber(String phoneNo) {
        this.number = phoneNo;
    }
    
    public String getType() {
        return type;
    }
    
    public void setType(String phoneType) {
        this.type = phoneType;
    }
    
    public Professor getProfessor() {
        return employee;
    }

    public void setProfessor(Professor employee) {
        this.employee = employee;
    }

    public String toString() {
        return "Phone id: " + getId() + 
               ", no: " + getNumber() +
               ", type: " + getType();
    }
}

以下代码来自Address.java。

package cn..common;

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

@Entity
public class Address {
    @Id
    private int id;
    private String street;
    private String city;
    private String state;
    private String zip;
    
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    public String getStreet() {
        return street;
    }
    
    public void setStreet(String address) {
        this.street = address;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getState() {
        return state;
    }

    public void setState(String state) {
        this.state = state;
    }

    public String getZip() {
        return zip;
    }

    public void setZip(String zip) {
        this.zip = zip;
    }
    public String toString() {
        return "Address id: " + getId() + 
               ", street: " + getStreet() +
               ", city: " + getCity() +
               ", state: " + getState() +
               ", zip: " + getZip();
    }

}

以下代码来自Professor.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.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
public class Professor {
    @Id
    private int id;
    private String name;
    private long salary;
    @Temporal(TemporalType.DATE)
    private Date startDate;
    
    @OneToOne
    private Address address;
    
    @OneToMany(mappedBy="employee")
    private Collection<Phone> phones = new ArrayList<Phone>();
    
    @ManyToOne
    private Department department;
    
    @ManyToOne
    private Professor manager;
    
    @OneToMany(mappedBy="manager")
    private Collection<Professor> directs = new ArrayList<Professor>();
    
    @ManyToMany(mappedBy="employees")
    private Collection<Project> projects = new ArrayList<Project>();

    public int getId() {
        return id;
    }
    
    public void setId(int empNo) {
        this.id = empNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }

    public long getSalary() {
        return salary;
    }

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

    public Date getStartDate() {
        return startDate;
    }

    public void setStartDate(Date startDate) {
        this.startDate = startDate;
    }
    
    public Collection<Phone> getPhones() {
        return phones;
    }
    
    public void addPhone(Phone phone) {
        if (!getPhones().contains(phone)) {
            getPhones().add(phone);
            if (phone.getProfessor() != null) {
                phone.getProfessor().getPhones().remove(phone);
            }
            phone.setProfessor(this);
        }
    }
    
    public Department getDepartment() {
        return department;
    }
    
    public void setDepartment(Department department) {
        if (this.department != null) {
            this.department.getProfessors().remove(this);
        }
        this.department = department;
        this.department.getProfessors().add(this);
    }
    
    public Collection<Professor> getDirects() {
        return directs;
    }
    
    public void addDirect(Professor employee) {
        if (!getDirects().contains(employee)) {
            getDirects().add(employee);
            if (employee.getManager() != null) {
                employee.getManager().getDirects().remove(employee);
            }
            employee.setManager(this);
        }
    }
    
    public Professor getManager() {
        return manager;
    }
    
    public void setManager(Professor manager) {
        this.manager = manager;
    }

    public Collection<Project> getProjects() {
        return projects;
    }
    
    public void addProject(Project project) {
        if (!getProjects().contains(project)) {
            getProjects().add(project);
        }
        if (!project.getProfessors().contains(this)) {
            project.getProfessors().add(this);
        }
    }
    
    public Address getAddress() {
        return address;
    }
    
    public void setAddress(Address address) {
        this.address = address; 
    }
    
    public String toString() {
        return "Professor " + getId() + 
               ": name: " + getName() +
               ", salary: " + getSalary() +
               ", phones: " + getPhones() +
               ", managerNo: " + ((getManager() == null) ? null : getManager().getId()) +
               ", deptNo: " + ((getDepartment() == null) ? null : getDepartment().getId());
    }

}

下面的代码来自Department.java。

package cn..common;

import java.util.HashSet;
import java.util.Set;

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 Set<Professor> employees = new HashSet<Professor>();

    public int getId() {
        return id;
    }
    
    public void setId(int deptNo) {
        this.id = deptNo;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String deptName) {
        this.name = deptName;
    }
    
    public Set<Professor> getProfessors() {
        return employees;
    }

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

下载 Query_Select_Two_Properties.zip

上面的代码生成以下结果。

以下是数据库转储。

Table Name: ADDRESS




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

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





Table Name: PHONE




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

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

    Column Name: SALARY,
    Column Type: BIGINT:
    Column Value: 0

    Column Name: STARTDATE,
    Column Type: DATE:
    Column Value: null

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

    Column Name: DEPARTMENT_ID,
    Column Type: INTEGER:
    Column Value: 1

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





Table Name: PROJECT




Table Name: PROJECT_PROFESSOR

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