如何修复使用 group by 解析 JPQL 的语法错误答案

【问题标题】：How to fix syntax error parsing JPQL with group by如何修复使用 group by 解析 JPQL 的语法错误
【发布时间】：2019-08-17 18:21:56
【问题描述】：

我是新人，在项目中使用 JPA 和 JPQL，但遇到 SQL 语法错误，找不到原因。

我正在使用 Spring Boot，MySQL。

我有一个投票库

public interface VoteRepository extends JpaRepository<Vote, Long> {
    ....

    @Query(value = "SELECT NEW com.self.polls.model.ChoiceVoteCount(v.choice.id, count(v.id)) FROM Vote v WHERE v.poll.id = :pollId GROUP BY v.choice.id", nativeQuery = true)
    List<ChoiceVoteCount> countByPollIdGroupByChoiceId(@Param("pollId") Long pollId);

    ....
}

选择投票数

public class ChoiceVoteCount {
     private Long choiceId;

    private Long voteCount;    
}

投票

public class Vote {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "poll_id", nullable = false)
    private Poll poll;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "choice_id", nullable = false)
    private Choice choice;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "user_id", nullable = false)
    private User user;
}

但运行此查询时出现错误

2019-03-27 16:01:34.783 ERROR 68064 --- [nio-5000-exec-4] o.h.engine.jdbc.spi.SqlExceptionHelper   : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '.self.polls.model.ChoiceVoteCount(v.choice.id, count(v.id)) FROM Vote v WHERE v.' at line 1
2019-03-27 16:01:34.787 ERROR 68064 --- [nio-5000-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.InvalidDataAccessResourceUsageException: could not extract ResultSet; SQL [n/a]; nested exception is org.hibernate.exception.SQLGrammarException: could not extract ResultSet] with root cause

java.sql.SQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '.self.polls.model.ChoiceVoteCount(v.choice.id, count(v.id)) FROM Vote v WHERE v.' at line 1

【问题讨论】：

删除nativeQuery = true，因为您使用的是jpql。
但是当我删除它时，我得到另一个错误```查询方法公共抽象java.util.Map com.self.polls.repository.VoteRepository.countByPollIdGroupByChoiceId()的验证失败！ ```当我启动应用程序时
对不起，我发现这是由类型错误引起的。它通过删除 nativeQuery 来显示。谢谢

标签： java jpa jpql

【解决方案1】：

您必须在 ChoiceVoteCount 中添加一个构造函数。
为了安全起见，还可以添加默认构造函数。

public ChoiceVoteCount() {
}

public ChoiceVoteCount(Long choiceId, Long voteCount) {
    this.choiceId = choiceId;
    this.voteCount = voteCount;
}

【讨论】：