mybatis-plus实现多表查询的示例代码

@Mapper
@Repository
public interface UserMapper extends BaseMapper<User> {
    @Select("select user.* ,area.area_name from user,area " +
            "where user.area_id = area.id and user.id = #{id}")
    User getUserById(int id);
}

<dependency>
    <groupId>com.github.yulichang</groupId>
    <artifactId>mybatis-plus-join</artifactId>
    <version>1.4.5</version>
</dependency>

import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.github.yulichang.wrapper.MPJLambdaWrapper;
import com.github.yulichang.toolkit.MPJQueryWrapper;
public class OrderService {
    @Autowired
    private OrderMapper orderMapper;
    public List<UserOrderDTO> getOrderWithUser() {
        MPJLambdaWrapper<Order> wrapper = new MPJLambdaWrapper<>();
        wrapper.selectAll(Order.class)        // 查询Order表所有字段
               .select(User::getName)         // 查询User表的Name字段
               .leftJoin(User.class, User::getId, Order::getUserId);  // 使用left join连接
        return orderMapper.selectJoinList(UserOrderDTO.class, wrapper);
    }
}

<mapper namespace="com.quanxiaoha.mybatisplusdemo.mapper.UserMapper">
    <resultMap id="orderMap" type="com.quanxiaoha.mybatisplusdemo.model.OrderVO">
        <result property="userName" column="name"/>
        <result property="userAge" column="age"/>
        <!-- 其他字段映射 -->
    </resultMap>
    <select id="selectOrders" resultMap="orderMap">
        select o.order_id, o.user_id, o.goods_name, o.goods_price, u.name, u.age
        from t_order as o left join t_user as u on o.user_id = u.id
    </select>
</mapper>