MySQL分組聚合group_concat + substr_index

場景:
給予一張商品售賣表,表中數據爲商品的售賣記錄,假設表中數據是定時腳本插入的,每一個時間段的商品售賣數量不一樣,根據此表找各個商品的最多售賣數量的數據

一、數據表html

CREATE TABLE `goods_sell` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `goods_id` int(10) unsigned NOT NULL DEFAULT '0',
  `sell_num` int(10) unsigned NOT NULL DEFAULT '0',
  `create_time` int(10) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

二、數據內容mysql

mysql> select * from goods_sell;
+----+----------+----------+-------------+
| id | goods_id | sell_num | create_time |
+----+----------+----------+-------------+
|  1 |        1 |        5 |  1420520010 |
|  2 |        2 |       10 |  1420520000 |
|  3 |        1 |       10 |  1410520000 |
|  4 |        1 |        5 |  1510520000 |
|  5 |        2 |        6 |  1510521000 |
|  6 |        3 |       15 |  1510621000 |
+----+----------+----------+-------------+
6 rows in set (0.00 sec)

剖析其要求,也就是說,要用1條sql
找出goods_id 爲1 的id爲4的數據
找出goods_id 爲2 的id爲2的數據
找出goods_id 爲3 的id爲6的數據sql

三、怎麼作呢?
這時就能夠用MySQL的分組聚合,GROUP_CONCAT和SUBSTRING_INDEX一塊兒使用。
#查找各個商品售賣最多的一條記錄,此時group_concat()中必定要order by排序。要否則截取第一個數據就不對了。ide

select id,goods_id,
SUBSTRING_INDEX(GROUP_CONCAT(sell_num order by sell_num desc),',',1) sell_num_max,
create_time 
from goods_sell group by goods_id order by create_time DESC;

+----+----------+--------------+-------------+
| id | goods_id | sell_num_max | create_time |
+----+----------+--------------+-------------+
|  6 |        3 | 15           |  1510621000 |
|  1 |        1 | 10           |  1420520010 |
|  2 |        2 | 10           |  1420520000 |
+----+----------+--------------+-------------+
3 rows in set (0.00 sec)

不使用SUBSTRING_INDEX的話,查出來的數據是:spa

select id,goods_id, GROUP_CONCAT(sell_num order by sell_num desc) sell_num_list, create_time  from good;

+----+----------+---------------+-------------+
| id | goods_id | sell_num_list | create_time |
+----+----------+---------------+-------------+
|  6 |        3 | 15            |  1510621000 |
|  1 |        1 | 10,5,5        |  1420520010 |
|  2 |        2 | 10,6          |  1420520000 |
+----+----------+---------------+-------------+
3 rows in set (0.00 sec)

因此用SUBSTRING_INDEX截取最前面的一個數據。

.net

文檔參考:
http://www.javashuo.com/article/p-ktqwnsme-ck.html
http://www.javashuo.com/article/p-zrgyqejy-hy.html
https://baijiahao.baidu.com/s?id=1595349117525189591&wfr=spider&for=pccode

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