Mybatis一对一，一对多关联查询

MyBatis是一款优秀的持久层框架，提供了灵活的SQL映射功能，支持复杂的数据库操作。本文将详细介绍MyBatis中一对一和一对多关联查询的实现。

一对一关联查询

一对一关联关系指的是一个表中的一条记录与另一个表中的一条记录相关联。例如，一个用户有一个地址信息。

数据库表设计

假设有两个表：user和 address。

CREATE TABLE user (
    id INT PRIMARY KEY,
    name VARCHAR(50)
);

CREATE TABLE address (
    id INT PRIMARY KEY,
    user_id INT,
    address VARCHAR(100),
    FOREIGN KEY (user_id) REFERENCES user(id)
);
​

实体类设计

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

    // Getters and setters
}

public class Address {
    private int id;
    private int userId;
    private String address;

    // Getters and setters
}
​

MyBatis Mapper XML配置

<mapper namespace="com.example.UserMapper">
    <resultMap id="userMap" type="User">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <association property="address" column="id" javaType="Address" select="selectAddressByUserId" />
    </resultMap>

    <select id="selectUserById" resultMap="userMap">
        SELECT * FROM user WHERE id = #{id}
    </select>

    <select id="selectAddressByUserId" resultType="Address">
        SELECT * FROM address WHERE user_id = #{id}
    </select>
</mapper>
​

接口Mapper

public interface UserMapper {
    User selectUserById(int id);
}
​

一对多关联查询

一对多关联关系指的是一个表中的一条记录与另一个表中的多条记录相关联。例如，一个订单有多个商品。

数据库表设计

假设有两个表：order和 item。

CREATE TABLE `order` (
    id INT PRIMARY KEY,
    order_number VARCHAR(50)
);

CREATE TABLE item (
    id INT PRIMARY KEY,
    order_id INT,
    item_name VARCHAR(50),
    FOREIGN KEY (order_id) REFERENCES `order`(id)
);
​

实体类设计

public class Order {
    private int id;
    private String orderNumber;
    private List<Item> items;

    // Getters and setters
}

public class Item {
    private int id;
    private int orderId;
    private String itemName;

    // Getters and setters
}
​

MyBatis Mapper XML配置

<mapper namespace="com.example.OrderMapper">
    <resultMap id="orderMap" type="Order">
        <id property="id" column="id" />
        <result property="orderNumber" column="order_number" />
        <collection property="items" ofType="Item" column="id" select="selectItemsByOrderId" />
    </resultMap>

    <select id="selectOrderById" resultMap="orderMap">
        SELECT * FROM `order` WHERE id = #{id}
    </select>

    <select id="selectItemsByOrderId" resultType="Item">
        SELECT * FROM item WHERE order_id = #{id}
    </select>
</mapper>
​

接口Mapper

public interface OrderMapper {
    Order selectOrderById(int id);
}
​

总结

MyBatis提供了强大的关联查询功能，通过配置 <association>和 <collection>标签，可以实现一对一和一对多的