如何使用JPA计算行数？

Question

我尝试使用JPA计算行数.我想使用where子句但是我不能.

CriteriaBuilder qb = entityManager.getCriteriaBuilder();
CriteriaQuery<Long> cq = qb.createQuery(Long.class);
cq.select(qb.count(cq.from(MyEntity.class)));
cq.where();  //how to write where clause 
return entityManager.createQuery(cq).getSingleResult();

如何设置where子句例如age ="45".提前致谢.

Answer 1

jpa 中最好和最简单的方法：

public interface YourEntityRepository extends JpaRepository<YourEntity, Serializable> {

Long countByAgeGreaterThan(Integer age);

}

Answer 2

使用ParameterExpression.注意:未经测试.

CriteriaBuilder qb = entityManager.getCriteriaBuilder();
CriteriaQuery<Long> cq = qb.createQuery(Long.class);
cq.select(qb.count(cq.from(MyEntity.class)));
ParameterExpression<Integer> p = qb.parameter(Integer.class);
q.where(qb.eq(c.get("age"), 45));
return entityManager.createQuery(cq).getSingleResult();

参考.

Answer 3

EntityManagerFactory emf =
                    Persistence.createEntityManagerFactory("your table name");
            EntityManager em = emf.createEntityManager();


            // JPA Query Language is executed on your entities (Java Classess), not on your database tables;

            Query query = em.createQuery("SELECT count(*) FROM your Classname WHERE ... etc");
            long count = (long) query.getSingleResult();