如何使用`COUNT`和`GROUP BY`编写JPQL查询

Question

如何使用COUNT和GROUP BY撰写 JPQL 查询，并将结果设为Map<Integer,Integer>？

public class CommentEntity {

 private int id;

 private int parentId;

 private EntityParentType parentType;

 private Long replyCounts;

 private String author;

 private String comment;

}

public enum EntityParentType {

 PERSON,
 EVENT,
 COMMENT;

}

我写了 MySQL 查询，这很好用：

SELECT parent_id, COUNT(*) FROM comment AS c WHERE c.parent_type = 2 AND c.parent_id IN (64,65) GROUP BY parent_id

但 JPQL 查询失败：

@Repository
@Transactional(readOnly = true)
public interface CommentRepository extends JpaRepository<CommentEntity, Integer> {

 @Query(value = "SELECT c.parentId, COUNT(c.id) FROM CommentEntity AS c WHERE c.parentType = ?1 AND c.parentId IN (?2) GROUP BY c.parentId")
 Map<Integer, Integer> findReplyCountByParentIds(EntityParentType entityParentType, List<Integer> ids);

}

。

Method threw 'org.springframework.dao.IncorrectResultSizeDataAccessException' exception.
result returns more than one elements

下面的

也是失败的：

@Query(value = "SELECT c.parentId, COUNT (c.id) FROM CommentEntity AS c WHERE c.parentType = ?1 AND c.parentId IN (?2) GROUP BY c.parentId")
 List<Map<Integer, Integer>> findReplyCountByParentIds(EntityParentType entityParentType, List<Integer> ids);

Method threw 'org.springframework.dao.InvalidDataAccessApiUsageException' exception.
No aliases found in result tuple! Make sure your query defines aliases!

我尝试将pacakge添加到CommentEntity，但也失败了

Answer 1

一种解决方法是使用构造函数语法，如下所示：

SELECT NEW org.apache.commons.lang3.tuple.ImmutablePair(c.parentId, COUNT(c.id)) FROM ...

当然，您可以使用任何其他类代替ImmutablePair（例如Map.MapEntry的具体实现）。然后，您将结果声明为List<ImmutablePair>，并将结果收集到服务方法中的地图中。