JPA Criteria SELECT条件

SELECT 子句用于从数据库中获取数据。可以以单表达式或多表达式的形式检索数据。在 Criteria API 中，每种形式的表达方式都不同。

JPA Criteria SELECT条件示例

通常，select() 方法用于 SELECT 子句以获取所有类型的表单。在这里，我们将对学生表执行几个 SELECT 操作。让我们假设该表包含以下记录：

StudentEntity实体类

package com.yiidian.pojo;

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

@Entity
@Table(name="student")
public class StudentEntity {  
  
    @Id
    private int s_id;  
    private String s_name;  
    private int s_age;  
      
    public StudentEntity(int s_id, String s_name, int s_age) {  
        super();  
        this.s_id = s_id;  
        this.s_name = s_name;  
        this.s_age = s_age;  
    }  
  
    public StudentEntity() {  
        super();  
    }  
  
    public int getS_id() {  
        return s_id;  
    }  
  
    public void setS_id(int s_id) {  
        this.s_id = s_id;  
    }  
  
    public String getS_name() {  
        return s_name;  
    }  
  
    public void setS_name(String s_name) {  
        this.s_name = s_name;  
    }  
  
    public int getS_age() {  
        return s_age;  
    }  
  
    public void setS_age(int s_age) {  
        this.s_age = s_age;  
    }
}  

persistence.xml文件

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
             xmlns="http://xmlns.jcp.org/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="Student_details">
        <class>com.yiidian.pojo.StudentEntity</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
                      value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                      value="jdbc:mysql://localhost:3306/jpa?serverTimezone=UTC&amp;useSSL=false" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="root" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

查询单条数据表达式

在这里，我们将借助一个简单的示例从数据库中获取单列。

package com.yiidian.test;

import com.yiidian.pojo.StudentEntity;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;
import java.util.List;

public class SingleFetch {

    public static void main(String args[]) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("Student_details");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);

        Root<StudentEntity> stud = cq.from(StudentEntity.class);

        cq.select(stud.get("s_name"));

        CriteriaQuery<StudentEntity> select = cq.select(stud);
        TypedQuery<StudentEntity> q = em.createQuery(select);
        List<StudentEntity> list = q.getResultList();

        System.out.println("s_id");

        for (StudentEntity s : list) {
            System.out.println(s.getS_id());
        }

        em.getTransaction().commit();
        em.close();
        emf.close();
    }
}  

输出结果为：

查询多条数据表达式

在这里，我们将借助一个简单示例从数据库中获取多列。

package com.yiidian.test;

import com.yiidian.pojo.StudentEntity;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;
import java.util.List;

public class MultiFetch {

    public static void main(String args[]) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("Student_details");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);

        Root<StudentEntity> stud = cq.from(StudentEntity.class);

        cq.multiselect(stud.get("s_id"), stud.get("s_name"), stud.get("s_age"));
        CriteriaQuery<StudentEntity> select = cq.select(stud);
        TypedQuery<StudentEntity> q = em.createQuery(select);
        List<StudentEntity> list = q.getResultList();

        System.out.print("s_id");
        System.out.print("\t s_name");
        System.out.println("\t s_age");

        for (StudentEntity s : list) {
            System.out.print(s.getS_id());
            System.out.print("\t" + s.getS_name());
            System.out.println("\t" + s.getS_age());
        }

        em.getTransaction().commit();
        em.close();
        emf.close();
    }
}  

输出结果为：