mysql中数据分组后取组内数据的最小值集合

问题:有n组数据,要取出所有组数据中某个字段值最小的那一条记录,最后得到一组数据,这组数据表示的是每个组的最小的那一条数据的集合?

示例:

+--------+------------+-------+
| type   | variety    | price |
+--------+------------+-------+
| apple  | gala       |  2.79 | 
| apple  | fuji       |  0.24 | 
| apple  | limbertwig |  2.87 | 
| orange | valencia   |  3.59 | 
| orange | navel      |  9.36 | 
| pear   | bradford   |  6.05 | 
| pear   | bartlett   |  2.14 | 
| cherry | bing       |  2.55 | 
| cherry | chelan     |  6.33 | 
+--------+------------+-------+

结果:

+--------+----------+-------+
| type   | variety  | price |
+--------+----------+-------+
| apple  | fuji     |  0.24 | 
| orange | valencia |  3.59 | 
| pear   | bartlett |  2.14 | 
| cherry | bing     |  2.55 | 
+--------+----------+-------+

注意:

不能直接group by 之后,取min值,这样分组值和min值是对的,但是无法保证otherMsg是对的,会发生混乱

select type, min(price) as minprice,otherMsg
   from fruits group by type

官方文档推荐的解决方案

1.使用一个不相关的子查询(Uncorrelated subquery)

SELECT s1.article, dealer, s1.price
FROM shop s1
JOIN (
  SELECT article, MAX(price) AS price
  FROM shop
  GROUP BY article) AS s2
  ON s1.article = s2.article AND s1.price = s2.price;

2.LEFT JOIN:自连接

SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price
WHERE s2.article IS NULL;

当 s1.price时最大值的时候,s2.article必然是null

---

查看博客的解决方案

1.内连接:查出每个组的最大值,然后连接查询

select f.type, f.variety, f.price
from (
   select type, min(price) as minprice
   from fruits group by type
) as x inner join fruits as f on f.type = x.type and f.price = x.minprice;

2.子查询

select type, variety, price
from fruits
where price = (select min(price) from fruits as f where f.type = fruits.type);

3.查找某组数据中最小的2条数据

select type, variety, price
from fruits
where (
   select count(*) from fruits as f
   where f.type = fruits.type and f.price <= fruits.price
) <= 2;

4.使用 UNION

(select * from fruits where type = 'apple' order by price limit 2)
union all
(select * from fruits where type = 'orange' order by price limit 2)
union all
(select * from fruits where type = 'pear' order by price limit 2)
union all
(select * from fruits where type = 'cherry' order by price limit 2)

5.先分组按照价格降序排序,然后对结果集再group by分组,让MySQL自动取第一个(存疑,性能问题+group by取值的随机性,虽然目前测试后确实是取的第一个,但官方文档没有说明group by是取第一个不保证日后这个机制还有效果)

SELECT * 
FROM (SELECT * 
FROM shop 
[WHERE conditions] 
ORDER BY price DESC) AS s 
GROUP BY article

6.本人使用的方法:查找到student_id,grade_id分组后得到的最大值count,然后将对应订单数据

SELECT o.`garde_id`, o.`student_id`, o.`count`,o.id
    FROM `wlx_order` o
    WHERE (o.`garde_id`, o.`student_id`, o.`count`) IN (SELECT `garde_id`, `student_id`, MAX(count)
            FROM `wlx_order`
            GROUP BY `garde_id`, `student_id`)
        AND o.`count` > 0
        AND o.`mark` = 1
        AND o.`status` = 'NORMAL';

---

查询回访记录中的最近时间的一个

select * from `wlx_visit_record` vc,`wlx_intention_student` is1 
where is1.`id`=vc.`intention_student_id` and vc.`creation_date` =(
select MAX(vc2.`creation_date`) from `wlx_visit_record` vc2 where vc2.`intention_student_id`=is1.id
);

参考博客:

https://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/

https://dev.mysql.com/doc/refman/5.7/en/example-maximum-column-group-row.html