Mybatisplus多表关联分页查询多种实现方式

@Data
public class User {
    private Long id;
    private String name;
}

@Data
public class Order {
    private Long id;
    private Long userId;
    private String productName;
}

import com.baomidou.mybatisplus.core.metadata.IPage;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import org.apache.ibatis.annotations.Param;

public interface UserMapper extends BaseMapper<User> {
    IPage<User> selectUserOrders(Page<?> page, @Param("userId") Long userId);
}

<select id="selectUserOrders" resultType="User">
    SELECT u.*, o.product_name
    FROM user u
    LEFT JOIN order o ON u.id = o.user_id
    WHERE u.id = #{userId}
</select>

@Autowired
private UserMapper userMapper;

public IPage<User> getUserOrders(Page<User> page, Long userId) {
    return userMapper.selectUserOrders(page, userId);
}

Page<User> page = new Page<>(1, 10); // 分页参数：第1页，每页10条
IPage<User> result = userService.getUserOrders(page, 1L);

@Select("SELECT u.*, o.product_name " +
        "FROM user u LEFT JOIN order o ON u.id = o.user_id " +
        "WHERE u.id = #{userId}")
IPage<User> selectUserOrdersCustom(Page<?> page, @Param("userId") Long userId);

Page<User> page = new Page<>(1, 10); // 分页参数
IPage<User> result = userMapper.selectUserOrdersCustom(page, 1L);

@Data
public class UserOrderDTO {
    private Long userId;
    private String userName;
    private String productName;
}

@Select("SELECT u.id AS userId, u.name AS userName, o.product_name AS productName " +
        "FROM user u LEFT JOIN order o ON u.id = o.user_id " +
        "WHERE u.id = #{userId}")
IPage<UserOrderDTO> selectUserOrderDTO(Page<?> page, @Param("userId") Long userId);

Page<UserOrderDTO> page = new Page<>(1, 10); // 分页参数
IPage<UserOrderDTO> result = userMapper.selectUserOrderDTO(page, 1L);

<dependency>
    <groupId>com.github.pagehelper</groupId>
    <artifactId>pagehelper-spring-boot-starter</artifactId>
    <version>1.2.13</version>
</dependency>

import com.github.pagehelper.PageHelper;
import com.github.pagehelper.PageInfo;

public PageInfo<UserOrderDTO> getUserOrdersPage(int pageNum, int pageSize, Long userId) {
    PageHelper.startPage(pageNum, pageSize);
    List<UserOrderDTO> list = userMapper.selectUserOrderList(userId);
    return new PageInfo<>(list);
}