JPA条件选择子句

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

条件选择示例

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：-

现在, 请按照以下步骤执行操作：-

• 创建一个实体类。在这里, 我们在com.srcmini.jpa包下创建了StudentEntity.java。此类包含三个属性s_id, s_name, s_age和所有必需的注释。

StudentEntity.java

package com.srcmini.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;
		}
		
	}

• 现在, 在Persistence.xml文件中映射实体类和其他数据库配置。

Persistence.xml

<persistence>
<persistence-unit name="Student_details">
   
      <class>com.srcmini.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/studentdata"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value=""/>
         <property name="eclipselink.logging.level" value="SEVERE"/>
         <property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
      </properties>
      
   </persistence-unit>

</persistence>

• 一旦创建了基本实体类, 并将配置映射到persistence.xml文件中, 就可以通过以下方式执行不同类型的选择操作：-

选择单个表达式

在这里, 我们将借助一个简单的示例从数据库中获取单个列。

SingleFetch.java

package com.srcmini.jpa.jpql;
import com.srcmini.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();  
	 }
}

输出：

选择多个表达式

在这里, 我们将借助一个简单的示例从数据库中获取多个列。

MultiFetch.java

package com.srcmini.jpa.jpql;
import com.srcmini.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();  
	 }
}

输出：