我如何在 Spring Boot 中编写连接

Question

我是spring boot的初学者，我正在研究数据库实体。 现在我处于一种情况，我需要在spring boot项目中使用join来从不同的表中获取数据。

我有 3 个表item 、 category和issued_item ，我为它编写了一个query以从中获取数据。

query是：

SELECT issued_item.issued_id,
       item.item_name,
       issued_item.issued_from,
       issued_item.issued_to,
       issued_item.quantity,
       category.cat_type,
       issued_item.issued_date
FROM issued_item
INNER JOIN item ON issued_item.ii_fk = item.item_id
INNER JOIN category ON item.ic_fk = category.cat_id;

现在我对如何在我的spring boot项目中编写它感到困惑。

此外，将其提升到一些高级水平，我使用parameters来访问以下items ：

SELECT issued_item.issued_id,
       item.item_name,
       issued_item.issued_from,
       issued_item.issued_to,
       issued_item.quantity,
       category.cat_type,
       issued_item.issued_date
FROM issued_item
INNER JOIN item ON issued_item.ii_fk = item.item_id
INNER JOIN category ON item.ic_fk = category.cat_id
WHERE issued_item.issued_date BETWEEN FromDate AND ToDate
ORDER BY issued_item.issued_date ASC;

FromDate和ToDate是两个不同的参数。

---

注意：如果有人需要实体代码，那么我会提供它。

在这种情况下使用dto类有用吗？

Answer 1

编写如下，使用返回类型为List<Object []> 。

@Query(value="SELECT issued_item.issued_id, 
item.item_name, issued_item.issued_from, 
issued_item.issued_to, issued_item.quantity, category.cat_type, 
issued_item.issued_date
FROM issued_item
INNER JOIN item
ON issued_item.ii_fk = item.item_id
INNER JOIN category
ON item.ic_fk = category.cat_id
WHERE issued_item.issued_date BETWEEN :fromdate AND 
:todate
ORDER BY issued_item.issued_date ASC", nativeQuery= true)
List<Object []> returnObject(@Param("fromdate" Date fromdate, @Param("todate") Date todate );

然后进行迭代获取值。

for(Object[] obj : result) {
 String issueId = (String) obj[0];
 //Get others variable also
}

Answer 2

实体类和存储库类应该类似于如下所示。 OneToMany 和 ManyToOne 向 JPA 提供主键/外键信息以连接表。 由于您有不遵循典型命名约定的表字段名称，因此必须通过 JoinColumn 显式命名它们。

一旦定义了实体，大多数查询都是由 JPA 自动生成的 - 您只需要以JPA 要求的格式提供方法声明

@Entity
@Table(name="item")
class Item{
  @Identity
  private Long item_id;

  ....other columns

  @OneToMany(fetch = FetchType.EAGER)
  private List<IssuedItem> issuedItems;

  @ManyToOne
  @JoinColumn(name="ic_fk")
  private Category category;
}

@Entity
@Table(name="category")
class Category{
  @Identity
  private Long cat_id;

  @OneToMany(fetch = FetchType.EAGER)
  private List<Item> items;

  ....other columns

  
}

@Entity
@Table(name="issued_item")
class IssuedItem{

  ....other columns

  
  @ManyToOne(fetch = FetchType.EAGER)
  @JoinColumn(name="ii_fk")
  private Item item;
}


@Repository
class IssuedItemRepository extends JpaRepository<IssuedItem, Long>{
  //implementation will be generated by JPA
  List<IssuedItem> findAllByIssuedDateBetween(
      Date FromDate,
      Date ToDate);
}

@Repository
class ItemRepository extends JpaRepository<Item, Long>{
}

@Repository
class CategoryRepository extends JpaRepository<Category, Long>{
}