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where =＆＃34; 45＆＃34;。提前谢谢。

Answer 1

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();

Reference

Answer 2

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();

Answer 3

jpa中最好，最简单的方法：

public interface YourEntityRepository extends JpaRepository<YourEntity, Serializable> {

Long countByAgeGreaterThan(Integer age);

}

Answer 4

你可以使用Query参数，示例：

StringBuilder strQuery ;
    strQuery= new StringBuilder();
    try {
        strQuery.append("FROM SSS s where s.age:age");
        Query query = this.getEntityManager().createQuery(
                strQuery.toString());
        query.setParameter("age", 45);
    } catch (Exception e) {
        LOGGER.info("Erreur", e);
    }

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如何使用JPA计算行数？

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