MyBatis中怎么进行多对多关联查询

在MyBatis中进行多对多关联查询可以通过使用<collection>和<association>标签来实现。

假设有两个实体类Author和Book，它们之间是多对多的关系，一个作者可以写多本书，一本书也可以由多个作者共同写作。首先需要在Author和Book的Mapper文件中配置关联查询的SQL语句：

<!-- AuthorMapper.xml -->
<select id="getAuthorById" resultType="Author" parameterType="int">
    SELECT * FROM author WHERE id = #{id}
</select>

<select id="getBooksByAuthorId" resultType="Book" parameterType="int">
    SELECT b.* FROM book b
    JOIN author_book ab ON b.id = ab.book_id
    WHERE ab.author_id = #{authorId}
</select>

<!-- BookMapper.xml -->
<select id="getBookById" resultType="Book" parameterType="int">
    SELECT * FROM book WHERE id = #{id}
</select>

<select id="getAuthorsByBookId" resultType="Author" parameterType="int">
    SELECT a.* FROM author a
    JOIN author_book ab ON a.id = ab.author_id
    WHERE ab.book_id = #{bookId}
</select>

然后在Author和Book实体类中定义关联的属性：

public class Author {
    private Integer id;
    private String name;
    private List<Book> books;
    
    // 省略getter和setter方法
}

public class Book {
    private Integer id;
    private String title;
    private List<Author> authors;
    
    // 省略getter和setter方法
}

最后在MyBatis配置文件中配置关联查询的映射：

<!-- AuthorMapper.xml -->
<resultMap id="AuthorResultMap" type="Author">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <collection property="books" ofType="Book" resultMap="BookResultMap" />
</resultMap>

<!-- BookMapper.xml -->
<resultMap id="BookResultMap" type="Book">
    <id property="id" column="id" />
    <result property="title" column="title" />
    <collection property="authors" ofType="Author" resultMap="AuthorResultMap" />
</resultMap>

通过以上配置，就可以在AuthorMapper和BookMapper中查询到多对多关联的数据，并且在返回结果中包含关联的实体对象列表。