把以上三张表联合起来用mybatis做一个增删改查

好的，首先需要定义一个对应表的Java实体类，如下所示：

public class User {
    private Integer id;
    private String name;
    private Integer age;
    private String email;
    // getter和setter方法省略
}

public class Order {
    private Integer id;
    private String orderNo;
    private Integer userId;
    // getter和setter方法省略
}

public class OrderDetail {
    private Integer id;
    private Integer orderId;
    private Integer productId;
    private Integer quantity;
    // getter和setter方法省略
}

然后，在Mybatis中定义对应的Mapper接口，如下所示：

public interface UserMapper {
    User selectUserById(Integer id);
    List<User> selectAllUsers();
    void insertUser(User user);
    void updateUser(User user);
    void deleteUser(Integer id);
}

public interface OrderMapper {
    Order selectOrderById(Integer id);
    List<Order> selectOrdersByUserId(Integer userId);
    void insertOrder(Order order);
    void updateOrder(Order order);
    void deleteOrder(Integer id);
}

public interface OrderDetailMapper {
    List<OrderDetail> selectOrderDetailsByOrderId(Integer orderId);
    void insertOrderDetail(OrderDetail orderDetail);
    void updateOrderDetail(OrderDetail orderDetail);
    void deleteOrderDetail(Integer id);
}

在Mybatis配置文件中，需要定义对应的Mapper映射关系，如下所示：

<mappers>
    <mapper resource="com/example/mapper/UserMapper.xml"/>
    <mapper resource="com/example/mapper/OrderMapper.xml"/>
    <mapper resource="com/example/mapper/OrderDetailMapper.xml"/>
</mappers>

然后，在对应的Mapper XML文件中，定义对应的SQL语句，如下所示：

<!-- UserMapper.xml -->
<mapper namespace="com.example.mapper.UserMapper">
    <select id="selectUserById" parameterType="java.lang.Integer" resultType="com.example.entity.User">
        SELECT id, name, age, email
        FROM user
        WHERE id = #{id}
    </select>

    <select id="selectAllUsers" resultType="com.example.entity.User">
        SELECT id, name, age, email
        FROM user
    </select>

    <insert id="insertUser" parameterType="com.example.entity.User">
        INSERT INTO user (name, age, email)
        VALUES (#{name}, #{age}, #{email})
    </insert>

    <update id="updateUser" parameterType="com.example.entity.User">
        UPDATE user
        SET name = #{name}, age = #{age}, email = #{email}
        WHERE id = #{id}
    </update>

    <delete id="deleteUser" parameterType="java.lang.Integer">
        DELETE FROM user
        WHERE id = #{id}
    </delete>
</mapper>

<!-- OrderMapper.xml -->
<mapper namespace="com.example.mapper.OrderMapper">
    <select id="selectOrderById" parameterType="java.lang.Integer" resultType="com.example.entity.Order">
        SELECT id, order_no, user_id
        FROM order
        WHERE id = #{id}
    </select>

    <select id="selectOrdersByUserId" parameterType="java.lang.Integer" resultType="com.example.entity.Order">
        SELECT id, order_no, user_id
        FROM order
        WHERE user_id = #{userId}
    </select>

    <insert id="insertOrder" parameterType="com.example.entity.Order">
        INSERT INTO order (order_no, user_id)
        VALUES (#{orderNo}, #{userId})
    </insert>

    <update id="updateOrder" parameterType="com.example.entity.Order">
        UPDATE order
        SET order_no = #{orderNo}, user_id = #{userId}
        WHERE id = #{id}
    </update>

    <delete id="deleteOrder" parameterType="java.lang.Integer">
        DELETE FROM order
        WHERE id = #{id}
    </delete>
</mapper>

<!-- OrderDetailMapper.xml -->
<mapper namespace="com.example.mapper.OrderDetailMapper">
    <select id="selectOrderDetailsByOrderId" parameterType="java.lang.Integer" resultType="com.example.entity.OrderDetail">
        SELECT id, order_id, product_id, quantity
        FROM order_detail
        WHERE order_id = #{orderId}
    </select>

    <insert id="insertOrderDetail" parameterType="com.example.entity.OrderDetail">
        INSERT INTO order_detail (order_id, product_id, quantity)
        VALUES (#{orderId}, #{productId}, #{quantity})
    </insert>

    <update id="updateOrderDetail" parameterType="com.example.entity.OrderDetail">
        UPDATE order_detail
        SET order_id = #{orderId}, product_id = #{productId}, quantity = #{quantity}
        WHERE id = #{id}
    </update>

    <delete id="deleteOrderDetail" parameterType="java.lang.Integer">
        DELETE FROM order_detail
        WHERE id = #{id}
    </delete>
</mapper>

最后，在Java程序中调用对应的Mapper接口方法，即可完成对应的增删改查操作，如下所示：

SqlSession sqlSession = sqlSessionFactory.openSession();

UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
User user = userMapper.selectUserById(1);
List<User> userList = userMapper.selectAllUsers();
user.setName("Tom");
userMapper.updateUser(user);
userMapper.deleteUser(1);

OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
Order order = orderMapper.selectOrderById(1);
List<Order> orderList = orderMapper.selectOrdersByUserId(1);
order.setOrderNo("20210315");
orderMapper.insertOrder(order);
orderMapper.updateOrder(order);
orderMapper.deleteOrder(1);

OrderDetailMapper orderDetailMapper = sqlSession.getMapper(OrderDetailMapper.class);
List<OrderDetail> orderDetailList = orderDetailMapper.selectOrderDetailsByOrderId(1);
orderDetailMapper.insertOrderDetail(orderDetail);
orderDetailMapper.updateOrderDetail(orderDetail);
orderDetailMapper.deleteOrderDetail(1);

sqlSession.commit();
sqlSession.close();