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。现在我需要在我的spring boot 项目中使用joins 从不同的表中获取数据。

我有 3 个表 item、category 和 issued_item，我写了一个 query 让它从中获取数据。

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;

现在我很困惑如何在我的spring boot 项目上写它。

此外，将其提高到一些高级水平，我使用parameters 访问items，例如：

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，我会提供。

在这种情况下使用dto 类有用吗？

Answer 1

如下所示，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。由于您的表字段名称不遵循典型的命名约定，因此必须通过 JoinColumn 显式命名它们。

一旦定义了实体，大多数查询将由 JPA 自动生成 - 您只需提供 JPA requires 格式的方法声明

@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>{
}