为什么spring data jpa不发布JOIN查询？

本文介绍了为什么spring data jpa不发布JOIN查询？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 29岁程序员，3月因学历无情被辞！ 我用一对多关系创建了两个实体 Book 和 Book_Category 。当我发行 BookCategoryRepository.findAll（）时，我期望hibernate使用'INNER JOIN'查询。但它只是发出查询来从Book_Category中获取数据。 我缺少什么？我应该怎么做才能让hibernate发出JOIN查询？ Book.java 公共类书籍{ @Id @GeneratedValue（strategy = GenerationType.AUTO） private int id; 私人字符串名称; @ManyToOne @JoinColumn（name =book_category_id）私人BookCategory bookCategory; } BookCategory.java @Entity @Table（name =book_category） public class BookCategory { @Id @GeneratedValue （strategy = GenerationType.AUTO） private int id; 私人字符串名称; @OneToMany（mappedBy =bookCategory，fetch = FetchType.EAGER，cascade = CascadeType.ALL） private Set< Book>图书; BookCategoryRepository.java public interface BookCategoryRepository扩展JpaRepository< BookCategory，Integer> {} bookCategoryRepository.findAll（） 解决方案 Hibernate默认使用第二个查询来检索子集合。其中一个原因是适当的限制查询。否则，结果集中的行数将比1方的实体数多，如果至少有一个孩子超过1个。 存在一个注释来改变hibernate中的这种行为，Spring Data Jpa Repositories忽略了这种行为。注释是 @Fetch（FetchMode.JOIN）。您可能会考虑 FetchMode如何在Spring Data JPA中工作如果你真的需要这种行为。 I have created two entities Book and Book_Category with one-to-many relationship. When I issued BookCategoryRepository.findAll(), I expected hibernate to use 'INNER JOIN' query. But it just issued query to take data from Book_Category.What I am missing? What should I do to make hibernate issue JOIN query?Book.java@Entitypublic class Book { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int id; private String name; @ManyToOne @JoinColumn(name = "book_category_id") private BookCategory bookCategory;}BookCategory.java@Entity@Table(name = "book_category")public class BookCategory { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int id; private String name; @OneToMany(mappedBy = "bookCategory", fetch = FetchType.EAGER, cascade = CascadeType.ALL) private Set<Book> books;}BookCategoryRepository.javapublic interface BookCategoryRepository extends JpaRepository<BookCategory, Integer> {}bookCategoryRepository.findAll() 解决方案 Hibernate uses by default a second query to retriev the child collection. One reason for this is a proper limit query. Otherwise, there would be more rows in the result set, than entities for the 1 side, if at least one has more than 1 child.There exists an annotation to change this behaviour in hibernate which is ignored by the Spring Data Jpa Repositories. The annotation is @Fetch(FetchMode.JOIN). You might consider How does the FetchMode work in Spring Data JPA if you really need this behaviour. 这篇关于为什么spring data jpa不发布JOIN查询？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！