Java 如何使用JPA计算行数？
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How to count number of rows using JPA?
提问by Harun ERGUL
I try to count number of rows using JPA.I want to use where clause however I can't.
我尝试使用 JPA 计算行数。我想使用 where 子句，但我不能。
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();
How can I set where clause forexample where age="45". Thanks in advance.
如何设置 where 子句，例如 where age="45"。提前致谢。
采纳答案by Buhake Sindi
Use ParameterExpression. Note: Untested.
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。
回答by RichardK
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();
回答by TinyOS
you can use the Query parameter, Example :
您可以使用查询参数，例如：
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);
}
it's useful for you?
对你有用吗？
回答by jdominus
The best and simple way in jpa :
jpa 中最好和最简单的方法：
public interface YourEntityRepository extends JpaRepository<YourEntity, Serializable> {
Long countByAgeGreaterThan(Integer age);
}
