WHERE子句用于在数据库上应用条件, 并根据该条件获取数据。在Criteria API中, AbstractQuery接口的where()方法用于设置条件。
条件示例 在这里, 我们将对学生表执行几个WHERE操作。让我们假设该表包含以下记录：

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现在, 请按照以下步骤执行操作：-

• 在com.srcmini.jpa包下创建一个实体类名称为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 {@Idprivate 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。

【JPA条件WHERE子句】Persistence.xml

< persistence> < persistence-unit name="Student_details"> < class> com.srcmini.jpa.StudentEntity< /class> < properties> < property name="javax.persistence.jdbc.driver" value="http://www.srcmini.com/com.mysql.jdbc.Driver"/> < property name="javax.persistence.jdbc.url" value="http://www.srcmini.com/jdbc:mysql://localhost:3306/studentdata"/> < property name="javax.persistence.jdbc.user" value="http://www.srcmini.com/root"/> < property name="javax.persistence.jdbc.password" value=""/> < property name="eclipselink.logging.level" value="http://www.srcmini.com/SEVERE"/> < property name="eclipselink.ddl-generation" value="http://www.srcmini.com/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：-

JPQL大于小于
Comparison.java

package com.srcmini.jpa.jpql; import com.srcmini.jpa.StudentEntity; import javax.persistence.*; import javax.persistence.criteria.*; import java.util.*; public class Comparison { public static void main( String args[]) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" ); EntityManager em = emf.createEntityManager(); em.getTransaction().begin( ); CriteriaBuilder cb=em.getCriteriaBuilder(); AbstractQuery< StudentEntity> cq1=cb.createQuery(StudentEntity.class); AbstractQuery< StudentEntity> cq2=cb.createQuery(StudentEntity.class); Root< StudentEntity> stud1=cq1.from(StudentEntity.class); cq1.where(cb.greaterThan(stud1.get("s_age"), 22)); CriteriaQuery< StudentEntity> select1 = ((CriteriaQuery< StudentEntity> ) cq1).select(stud1); TypedQuery< StudentEntity> tq1 = em.createQuery(select1); List< StudentEntity> list1 = tq1.getResultList(); System.out.println("Students having age greater than 22"); System.out.print("s_id"); System.out.print("\t s_name"); System.out.println("\t s_age"); for(StudentEntity s:list1){System.out.print(s.getS_id()); System.out.print("\t"+s.getS_name()); System.out.println("\t"+s.getS_age()); }Root< StudentEntity> stud2=cq2.from(StudentEntity.class); cq2.where(cb.lessThan(stud2.get("s_age"), 22)); CriteriaQuery< StudentEntity> select2 = ((CriteriaQuery< StudentEntity> ) cq2).select(stud2); TypedQuery< StudentEntity> tq2 = em.createQuery(select2); List< StudentEntity> list2 = tq2.getResultList(); System.out.println("Students having age Less than 22"); System.out.print("s_id"); System.out.print("\t s_name"); System.out.println("\t s_age"); for(StudentEntity s:list2){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：

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JPQL之间
Between.java

package com.srcmini.jpa.jpql; import com.srcmini.jpa.StudentEntity; import javax.persistence.*; import javax.persistence.criteria.*; import java.util.*; public class Between { public static void main( String args[]) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" ); EntityManager em = emf.createEntityManager(); em.getTransaction().begin( ); CriteriaBuilder cb=em.getCriteriaBuilder(); AbstractQuery< StudentEntity> cq=cb.createQuery(StudentEntity.class); Root< StudentEntity> stud=cq.from(StudentEntity.class); cq.where(cb.between(stud.get("s_age"), 22, 26)) ; CriteriaQuery< StudentEntity> select = ((CriteriaQuery< StudentEntity> ) cq).select(stud); TypedQuery< StudentEntity> tq = em.createQuery(select); List< StudentEntity> list = tq.getResultList(); System.out.println("Students having age between 22 and 26"); 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：

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JPQL喜欢
Like.java

package com.srcmini.jpa.jpql; import com.srcmini.jpa.StudentEntity; import javax.persistence.*; import javax.persistence.criteria.*; import java.util.*; public class Like { public static void main( String args[]) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" ); EntityManager em = emf.createEntityManager(); em.getTransaction().begin( ); CriteriaBuilder cb=em.getCriteriaBuilder(); AbstractQuery< StudentEntity> cq=cb.createQuery(StudentEntity.class); Root< StudentEntity> stud=cq.from(StudentEntity.class); cq.where(cb.like(stud.get("s_name"), "R%")); CriteriaQuery< StudentEntity> select = ((CriteriaQuery< StudentEntity> ) cq).select(stud); TypedQuery< StudentEntity> tq = em.createQuery(select); List< StudentEntity> list = tq.getResultList(); System.out.println("Students name starting with R"); 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：

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JPQL输入
In.java

package com.srcmini.jpa.jpql; import com.srcmini.jpa.StudentEntity; import javax.persistence.*; import javax.persistence.criteria.*; import java.util.*; public class In.java { public static void main( String args[]) {EntityManagerFactory emf = Persistence.createEntityManagerFactory( "Student_details" ); EntityManager em = emf.createEntityManager(); em.getTransaction().begin( ); CriteriaBuilder cb=em.getCriteriaBuilder(); AbstractQuery< StudentEntity> cq=cb.createQuery(StudentEntity.class); Root< StudentEntity> stud=cq.from(StudentEntity.class); cq.where(cb.in(stud.get("s_age")).value(22).value(24)); CriteriaQuery< StudentEntity> select = ((CriteriaQuery< StudentEntity> ) cq).select(stud); TypedQuery< StudentEntity> tq = em.createQuery(select); List< StudentEntity> list = tq.getResultList(); System.out.println("Students having age 22 and 24"); 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：

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