Java JPA 标准API使用

Posted 2023-02-02 程序员JohnDeng

一.创建条件查询的步骤

1. 第一步通过EntityManager接口实例调用getCriteriaBuilder（）方法创建CriteriaBuilder接口的对象

EntityManager em = emf.createEntityManager();
CriteriaBuilder cb= em.getCriteriaBuilder();

2. 第二步构造一个CriteriaQuery接口实例来创建一个查询对象

CriteriaQuery<StudentEntity> cq =cb.createQuery(StudentEntity.class);

3. 第三步从CriteriaQuery对象方法调用来设置查询的根

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

4. 第四步调用CriteriaQuery对象的select（）方法指定查询结果集类型

CriteriaQuery<StudentEntity> select =cq.select(rootStudent);

5. 第五步创建Query接口实例指定访问数据库记录的方法类型

Query query=em.createQuery(select);

6. 第六步通过查询接口的方法来控制查询的执行

List<StudentEntity> list= query.getResultList();

二.标准API查询子句的方法

字句	Criteria API接口	方法
SELECT	CriteriaQuery	select()
FROM	AbstractQuery	from()
WHERE	AbstractQuery	where()
ORDER BY	CriteriaQuery	orderBy()
GROUP BY	AbstractQuery	groupBy()
HAVING	AbstractQuery	having()

三.举例说明查询代码实例

• 查询学生名称列表（一个列名）

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);
cq.select(stud.get("name"));
CriteriaQuery<StudentEntity> select = cq.select(stud);
TypedQuery<StudentEntity> query = em.createQuery(select);
List<StudentEntity> list = query.getResultList();

• 查询名称，年龄，性别学生列表(多个列名）

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.multiselect(stud.get("name"),stud.get("age"),stud.get("sex"));

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

• 根据名称排序(升序）查询学生列表(orderBy)

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.orderBy(cb.asc(stud.get("name"));

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

• 根据学生名称模糊查询列表(like)

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
AbstractQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.where(cb.like(stud.get("name"),"%");

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

• 根据多个学生ID查询列表（in）

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
AbstractQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.where(cb.in(stud.get("id")).value("1001").value("1002").value("1003");

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

• 查询10到18岁的学生列表（between）

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
AbstractQuery<StudentEntity> cq = cb.createQuery(StudentEntity.class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.where(cb.between(stud.get("age"),10,18));

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

• 统计每个年龄段的学生有多少个(groupBy)

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();

CriteriaQuery<Object[]> cq = cb.createQuery(Object[].class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.multiselect(stud.get("age"), cb.count(stud)).groupBy(stud.get("age"));

List<Object[]> list = em.createQuery(cq).getResultList();

• 统计24岁年龄段的学生有多少个

EntityManagerFactory emf =Persistence.createEntityManagerFactory("student");
EntityManager em= emf.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();

CriteriaQuery<Object[]> cq = cb.createQuery(Object[].class);
Root<StudentEntity> stud = cq.from(StudentEntity.class);

cq.multiselect(stud.get("age"), cb.count(stud)).groupBy(stud.get("age"))
			.having(cb.ge(stud.get("age"), 24));

List<Object[]> list = em.createQuery(cq).getResultList();