mybatis collection

MyBatis中的Collection指的是一种对象属性映射关系，即将Java对象中的集合属性与数据库表中的关联表进行映射。这种映射关系可以在MyBatis的mapper文件中使用<select/>元素来实现。在这个元素中，可以使用<collection/>子元素来配置Java对象中集合属性与数据库中关联表之间的映射关系，从而完成数据的查询和操作。

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mybatis collection

Mybatis collection是Mybatis框架中的一种特殊的映射方式，它可以用于处理Java集合类型的属性，例如List、Set、Map等。通过使用Mybatis collection，我们可以在一次查询中获取多个相关联的数据，而不需要进行多次查询。在Mybatis中，collection可以通过嵌套查询或者嵌套结果映射来实现。嵌套查询是指在主查询中使用子查询来获取相关联的数据，而嵌套结果映射是指在主查询中使用嵌套的结果映射来获取相关联的数据。Mybatis collection的使用可以大大简化我们的代码，提高查询效率。

mybatis collection 示例

Sure, here's an example of using MyBatis collection in Java:

Suppose we have two classes, Blog and Post, where a Blog can have multiple Posts. We want to retrieve all the Blogs and their respective Posts using MyBatis.

First, we need to define the mappings for the two classes in the MyBatis XML configuration file:

<resultMap id="blogMap" type="Blog">
  <id property="id" column="id" />
  <result property="name" column="name" />
  <collection property="posts" ofType="Post">
    <id property="id" column="post_id" />
    <result property="title" column="title" />
    <result property="content" column="content" />
  </collection>
</resultMap>

<resultMap id="postMap" type="Post">
  <id property="id" column="id" />
  <result property="title" column="title" />
  <result property="content" column="content" />
</resultMap>

In the first resultMap, we define the mapping for the Blog class and its collection of Posts. In the collection element, we specify the property to which the collection should be mapped (in this case, "posts"), the type of the collection element (Post), and the mappings for the collection element.

In the second resultMap, we define the mapping for the Post class.

Next, we can define the SQL statement to retrieve the data:

<select id="getAllBlogs" resultMap="blogMap">
  SELECT b.id, b.name, p.id AS post_id, p.title, p.content
  FROM blog b
  LEFT JOIN post p ON b.id = p.blog_id
  ORDER BY b.id ASC
</select>

Finally, we can use the MyBatis mapper interface to call the SQL statement and retrieve the data:

public interface BlogMapper {
  @Select("getAllBlogs")
  @ResultMap("blogMap")
  List<Blog> getAllBlogs();
}

With this setup, calling the getAllBlogs() method on the BlogMapper interface will return a List of Blogs, each with its collection of Posts populated.