能提供一个MyBatis中实现一对一或多对多关联映射的具体教程案例吗？

在MyBatis中，实现一对一或多对多关联映射通常涉及SQL方言（如XML配置文件或注解）、 resultMap 和 selectManyMap 等元素。这里我会给出一个简单的例子：

### 一对一关联映射

假设我们有两个实体：User 和 Address，用户有一个唯一的Address。

1. 定义实体类：

public class User {
    private Integer id;
    private String name;
    private Address address;

    // getters and setters
}

public class Address {
    private Integer id;
    private String street;

    // getters and setters
}

2. MyBatis XML 映射文件 (UserMapper.xml):

<mapper namespace="com.example.UserMapper">
    <resultMap id="UserResult" type="User">
        <id property="id" column="user_id"/>
        <result property="name" column="user_name"/>
        <association property="address" javaType="Address">
            <selectKey keyProperty="id" resultType="Integer" order="AFTER">
                SELECT LAST_INSERT_ID() AS id
            </selectKey>
            <result property="street" column="address_street"/>
        </association>
    </resultMap>

    <!-- 用户查询其地址 -->
    <select id="getUserWithAddress" resultMap="UserResult">
        SELECT * FROM user WHERE id = #{id}
    </select>
</mapper>

3. 使用时，注入 UserMapper 调用 `getUserWithAddress` 查询：

UserMapper mapper = new SqlSessionTemplate(sessionFactory.openSession()).getMapper(UserMapper.class);
User user = mapper.getUserWithAddress(userId);

### 多对多关联映射（通过关联表）

假设有 Products 和 Categories，一个产品可以有多类别，一个类别也可以包含多个产品，中间表为 product_category。

1. Entity 类

public class Product {
    private Integer id;
    private String name;
    private List<Category> categories; // ManyToOne relationship

    // getters and setters
}

public class Category {
    private Integer id;
    private String name;
    private List<Product> products; // ManyToOne relationship

    // getters and setters
}

2. 映射文件（ProductMapper.xml）

<select id="findProductsByCategoryIds" parameterType="List<Integer>" resultMap="ProductResult">
    SELECT p.* FROM product p
    LEFT JOIN product_category pc ON p.id = pc.product_id
    LEFT JOIN category c ON pc.category_id = c.id
    WHERE c.id IN
    <foreach item="categoryId" index="index" collection="categoryIds" open="(" separator="," close=")">
        #{categoryId}
    </foreach>
</select>
<resultMap id="ProductResult" type="Product">
    <!-- ... -->
    <collection property="categories" ofType="Category">
        <id property="id" column="category_id"/>
        <result property="name" column="category_name"/>
    </collection>
</resultMap>

使用时，调用 `findProductsByCategoryIds`：

List<Integer> categoryIds = ...;
List<Product> products = mapper.findProductsByCategoryIds(categoryIds);