JPA标准SELECT子句

SELECT子句用于从数据库中获取数据。 数据可以以单个表达式或多个表达式的形式检索。 在Criteria API中，每个表单都有不同的表达。

标准SELECT示例

通常，select()方法 SELECT子句来获取所有类型表单。 在这里，我们将在student表上执行几个SELECT操作。 假设该表包含以下记录 -

DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `S_ID` int(11) NOT NULL,
  `S_NAME` varchar(255) DEFAULT NULL,
  `S_AGE` int(11) DEFAULT NULL,
  PRIMARY KEY (`S_ID`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('101', 'Gaurav', '24');
INSERT INTO `student` VALUES ('102', 'Rahul', '22');
INSERT INTO `student` VALUES ('103', 'Chris', '20');
INSERT INTO `student` VALUES ('104', 'Ronit', '26');
INSERT INTO `student` VALUES ('105', 'Roy', '21');

现在，请按照以下步骤执行操作: -

第1步: 创建一个实体类。在com.zyiz.jpa包下创建了StudentEntity.java类文件。 该类包含三个属性:s_id，s_name，s_age以及所有必需的注解。

文件:StudentEntity.java -

package com.zyiz.jpa;

import javax.persistence.*;

@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;
    }

}

第2步: 将实体类和其他数据库配置映射到 persistence.xml 文件中。

文件:Persistence.xml -

<?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.zyiz.jpa.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/testdb?serverTimezone=UTC" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password"
                value="123456" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation"
                value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

这里，我们创建了基本实体类并将配置映射到persistence.xml 文件中，可以通过以下方式执行不同类型的SELECT操作 -

1. 选择单个表达式

在这里，我们将通过一个简单的示例从数据库中获取单列值。

文件:SingleFetch.java -

package com.zyiz.jpa.jpql;

import com.zyiz.jpa.StudentEntity;
import javax.persistence.*;
import javax.persistence.criteria.*;

import java.util.*;

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();
    }
}

执行上面代码，得到以下输出结果 -

s_id


104

2. 选择多个表达式

在这里，我们将通过一个简单的例子从数据库中获取多列的值。

文件:MultiFetch.java -

package com.zyiz.jpa.jpql;

import com.zyiz.jpa.StudentEntity;
import javax.persistence.*;
import javax.persistence.criteria.*;

import java.util.*;

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();
    }
}

执行上面代码，得到以下输出结果 -

s_id     s_name     s_age
   Gaurav    24
   Rahul    22
   Chris    20
   Ronit    26
   Roy    21