如何使用JPA和CriteriaBuilder按结果对组进行计数？

Question

我认为这几乎是不可能或非常棘手的。我正在使用CriteriaBuilder，JPA 2.0，Hibernate和MariaDB，并希望使用CriteriaBuilder构建以下查询：

SELECT COUNT(*) FROM
   (SELECT DISTINCT(SomeColumn) // I think this is not possible?
    FROM MyTable
    WHERE ... COMPLEX CLAUSE ...
    GROUP BY SomeColumn) MyTable

我的问题：可能吗？如果，怎么办？

感谢您对此全神贯注！

标记

Answer 1

此示例假定您正在使用元模型生成。

CriteriaQuery<Long> cq = cb.createQuery(Long.class);
Subquery<SomeColumnType> subcq = cq.subquery(SomeColumnType.class);
Root<MyTable> from = subcq.from(MyTable.class);
subcq.select(from.get(MyTable_.someColumn));
subcq.where(** complex where statements **);
subcq.groupBy(from.get(MyTable_.someColumn));
cq.select(cb.count(subcq));

Answer 2

[如果您有Spring Boot并且想要相同，还有一个更好的解决方法，通过使用本机SQL_CALC_FOUND_ROWS和FOUND_ROWS，该方法仅适用于MySql / Maria；

此集成的主要思想是Spring-Data的灵活性以及如下覆盖默认JpaRepositoryFactoryBean的能力

步骤1-创建自定义CustomJpaRepositoryFactoryBean：

/**
* 
* @author ehakawati
*
* @param <R>
* @param <T>
* @param <I>
*/
public class CustomJpaRepositoryFactoryBean<R extends JpaRepository<T, I>, T, I extends Serializable>
        extends JpaRepositoryFactoryBean<R, T, I> {

    public CustomJpaRepositoryFactoryBean(Class<? extends R> repositoryInterface) {
        super(repositoryInterface);
    }

    @Override
    protected RepositoryFactorySupport createRepositoryFactory(EntityManager entityManager) {
        RepositoryFactorySupport repositoryFactorySupport = super.createRepositoryFactory(entityManager);
        repositoryFactorySupport.setRepositoryBaseClass(CustomJpaRepository.class);
        return repositoryFactorySupport;
    }

}

步骤2-创建自定义CustomJpaRepository

 /**
 * 
 * @author ehakawati
 *
 * @param <T>
 * @param <ID>
 */
public class CustomJpaRepository<T, ID extends Serializable> extends SimpleJpaRepository<T, ID>
        implements Repository<T, ID> {

    private final EntityManager em;

    /**
     * 
     * @param entityInformation
     * @param entityManager
     */
    public CustomJpaRepository(JpaEntityInformation<T, ?> entityInformation, EntityManager entityManager) {
        super(entityInformation, entityManager);
        this.em = entityManager;

    }

    /**
     * 
     * @param domainClass
     * @param entityManager
     */
    public CustomJpaRepository(Class<T> domainClass, EntityManager entityManager) {
        super(domainClass, entityManager);
        this.em = entityManager;
    }

    /**
     * Reads the given {@link TypedQuery} into a {@link Page} applying the given
     * {@link Pageable} and {@link Specification}.
     *
     * @param query       must not be {@literal null}.
     * @param domainClass must not be {@literal null}.
     * @param spec        can be {@literal null}.
     * @param pageable    can be {@literal null}.
     * @return
     */
    protected <S extends T> Page<S> readPage(TypedQuery<S> query, final Class<S> domainClass, Pageable pageable,
            @Nullable Specification<S> spec) {

        if (pageable.isUnpaged()) {
            return super.readPage(query, domainClass, pageable, spec);
        }

        query.setFirstResult((int) pageable.getOffset());
        query.setMaxResults(pageable.getPageSize());

        return PageableExecutionUtils.getPage(query.getResultList(), pageable, () -> getNativeCount());
    }

    /**
     * 
     */

    protected long getNativeCount() {

        final Query query = em.createNativeQuery("SELECT FOUND_ROWS() as `count`");

        return ((BigInteger) query.getSingleResult()).longValue();

    }

}

步骤3-PageableQueriesInterceptor：

public class PageableQueriesInterceptor extends EmptyInterceptor {

    private static final Pattern PATTERN = Pattern.compile(".*?limit \\?(, \\?)?$");
    private static final long serialVersionUID = 1L;

    @Override
    public String onPrepareStatement(String sql) {

        if (PATTERN.matcher(sql).find()) {
            sql = sql.replaceFirst("select", "select SQL_CALC_FOUND_ROWS ");
        }

        return sql;
    }
}

第4步-启用PageableQueriesInterceptor：

sprint.jpa.properties.hibernate.ejb.interceptor= com.****.****.******.betterpaging.PageableQueriesInterceptor

享受