根据用户选择最后一个学习（最大）课程

Question

我在做什么？

我根据研究显示用户。 我想获取每个用户研究的最后一门课程。

问题？

根据最后学习的课程，我没有得到最后一个课程名称。我正在获得第一个课程名称。（java）但我希望（oracle）作为用户最后学习的课程。

用户表

user_id | Name
====================
1       | Zishan                                          
2       | Ellen

课程表

course_id | course_name | user_id | course_year 
==================================================
   1      |  java       | 1       | 2015   
   2      |  C++        | 1       | 2017
   3      |  oracle     | 1       | 2016
   4      |  dot net    | 2       | 2016

结果表

Name    | last_course_name | last_course_year
============================================
Zishan  | java             | 2017
Ellen   | dot net          | 2016

预期结果

Name    | last_course_name | last_course_year
============================================
Zishan  | C++              | 2017
Ellen   | dot net          | 2016

查询

SELECT `u`.`name`, MAX(`c`.`course_year`) as last_course_year , `c`.`course_name` as last_course_name 
FROM `user` as `u`
LEFT OUTER JOIN `course` as `c` ON `u`.`id` = `c`.`user_id`
GROUP BY `u`.`id`

Active Record Query：

$this->db->select('u.name','c.course_name as last_course_name');
$this->db->select_max('c.course_year as last_course_year'); 
$this->db->from('user as u');
$this->db->join('course as c', 'u.id = c.user_id', 'left');
$this->db->join('course as c1', 'c.user_id = c1.user_id', 'left outer');    
$this->db->where('c1.user_id IS NULL', null, false); 
$this->db->group_by('u.id');
$user_couse_data_query = $this->db->get();

Answer 1

您的查询不起作用，因为您需要在group by中提供未应用聚合函数的所有列。

试试这个：

select c.name,b.last_course_name,a.last_course_year
from
(select user_id,max(course_year) as last_course_year 
from course_table
group by user_id) a
left join
course_table b
on a.user_id = b.user_id and a.course_year = b.course_year
left join
user_table c
on a.user_id = c.user_id;  

如有任何疑问，请与我联系。

Answer 2

在这里，您可以从课程表中找到每个学生的最新一行

SELECT `u`.`name`, `c`.`course_name` as last_course_name ,c.course_year
FROM `user` as `u`
LEFT JOIN `course` as `c` ON `u`.`id` = `c`.`user_id`
LEFT JOIN `course` as `c1` ON `c`.`user_id` = `c1`.`user_id` AND  `c`.`course_year` < `c1`.`course_year`
WHERE `c1`.`user_id` is null

Demo

  

请注意，如果他们共享同一年，可能会返回多个课程

等效的活动记录查询将类似于

$this->db->select('u.name','c.course_name as last_course_name','c.course_year as last_course_year');
$this->db->from('user as u');
$this->db->join('course as c', 'u.id = c.user_id', 'left');
$this->db->join('course as c1', 'c.user_id = c1.user_id AND c.course_year < c1.course_year', 'left');    
$this->db->where('c1.user_id IS NULL', null, false); 
$user_couse_data_query = $this->db->get();

Answer 3

试试这个;

select U.Name, C.course_name as last_course_name, C2.maxCourseYear as last_course_year from User U 
inner join
( select user_id,max(course_year) as maxCourseYear from Course C
group by C.user_id) C 
ON U.user_id = C.user_id
inner join Course C2 ON C2.course_year = C.maxCourseYear

Answer 4

使用以下子查询

SELECT u.name, 
       c.course_year as last_course_year, 
       c.course_name as last_course_name 
FROM user as u
LEFT OUTER JOIN course as c ON u.id = c.user_id and 
                               (c.user_id, c.course_year) IN
               (
                  SELECT c.user_id,  MAX(c.course_year)
                  FROM course as c 
                  GROUP BY c.user_id
                )