JPA標準SELECT子句
SELECT子句用於從數據庫中獲取數據。 數據可以以單個表達式或多個表達式的形式檢索。 在Criteria API中，每個表單都有不同的表達。
標準SELECT示例
通常，select()方法 SELECT子句來獲取所有類型表單。 在這裏，我們將在student表上執行幾個SELECT操作。 假設該表包含以下記錄 -
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');現在，請按照以下步驟執行操作: -
第1步: 創建一個實體類。在com.yiibai.jpa包下創建了StudentEntity.java類文件。 該類包含三個屬性:s_id，s_name，s_age以及所有必需的註解。
文件:StudentEntity.java -
package com.yiibai.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: 將實體類和其他數據庫配置映射到 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.yiibai.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>這裏，我們創建了基本實體類並將配置映射到persistence.xml 文件中，可以通過以下方式執行不同類型的SELECT操作 -
1. 選擇單個表達式
在這裏，我們將通過一個簡單的示例從數據庫中獲取單列值。
文件:SingleFetch.java -
package com.yiibai.jpa.jpql;
import com.yiibai.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();
}
}執行上面代碼，得到以下輸出結果 -
s_id
101
102
103
104
1052. 選擇多個表達式
在這裏，我們將通過一個簡單的例子從數據庫中獲取多列的值。
文件:MultiFetch.java -
package com.yiibai.jpa.jpql;
import com.yiibai.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();
}
}執行上面代碼，得到以下輸出結果 -
s_id s_name s_age
101 Gaurav 24
102 Rahul 22
103 Chris 20
104 Ronit 26
105 Roy 21