MyBatis 的表关联的映射？

在 MyBatis 中进行表关联的映射，可以通过 SQL 语句和结果映射来实现。以下是一对一和一对多关联的映射示例：

一对一关联：

假设有两张表：users 和 user_details，它们之间通过用户ID关联。

1. XML 映射文件方式：

<!-- UserMapper.xml -->
<select id="getUserWithDetailsById" resultMap="UserWithDetailsResultMap">
    SELECT u.*, d.address, d.phone 
    FROM users u 
    LEFT JOIN user_details d ON u.id = d.user_id 
    WHERE u.id = #{id}
</select>

<resultMap id="UserWithDetailsResultMap" type="User">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <!-- Other properties from users table -->

    <association property="details" javaType="UserDetails">
        <result property="address" column="address"/>
        <result property="phone" column="phone"/>
        <!-- Other properties from user_details table -->
    </association>
</resultMap>

2. 注解方式：

public class User {
    private int id;
    private String name;
    // Other properties

    private UserDetails details;
    // Getter and Setter
}

public class UserDetails {
    private String address;
    private String phone;
    // Other properties
    // Getter and Setter
}

@Results({
    @Result(property = "id", column = "id"),
    @Result(property = "name", column = "name"),
    // Other results from users table

    @Result(property = "details.address", column = "address"),
    @Result(property = "details.phone", column = "phone")
    // Other results from user_details table
})
@Select("SELECT u.*, d.address, d.phone FROM users u LEFT JOIN user_details d ON u.id = d.user_id WHERE u.id = #{id}")
User getUserWithDetailsById(int id);

一对多关联：

假设有两张表：users 和 orders，一个用户可以有多个订单。

1. XML 映射文件方式：

<!-- UserMapper.xml -->
<select id="getUserWithOrdersById" resultMap="UserWithOrdersResultMap">
    SELECT u.*, o.order_id, o.order_date, o.amount
    FROM users u
    LEFT JOIN orders o ON u.id = o.user_id
    WHERE u.id = #{id}
</select>

<resultMap id="UserWithOrdersResultMap" type="User">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <!-- Other properties from users table -->

    <collection property="orders" ofType="Order">
        <id property="orderId" column="order_id"/>
        <result property="orderDate" column="order_date"/>
        <result property="amount" column="amount"/>
        <!-- Other properties from orders table -->
    </collection>
</resultMap>

2. 注解方式：

public class User {
    private int id;
    private String name;
    // Other properties

    private List<Order> orders;
    // Getter and Setter
}

public class Order {
    private int orderId;
    private Date orderDate;
    private double amount;
    // Other properties
    // Getter and Setter
}

@Results({
    @Result(property = "id", column = "id"),
    @Result(property = "name", column = "name"),
    // Other results from users table

    @Result(property = "orders", column = "id", javaType = List.class,
        many = @Many(select = "getOrdersByUserId"))
})
@Select("SELECT * FROM users WHERE id = #{id}")
User getUserWithOrdersById(int id);

@Select("SELECT * FROM orders WHERE user_id = #{userId}")
List<Order> getOrdersByUserId(int userId);

以上展示了一对一和一对多的表关联映射方式，通过 MyBatis 的结果映射机制，可以方便地处理多表关联查询，并将结果映射到 Java 对象中。

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