MySQL运维实战系列：MySQL5.7 Group By 问题

一、环境

MySQL版本：MySQL5.7.22

表结构：

CREATE TABLE `crm_report_accounting_income` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `contract_id` int(10) NOT NULL,
  `contract_no` varchar(50) NOT NULL,
  `date` int(8) NOT NULL,
  `city_id` int(11) NOT NULL DEFAULT '0' COMMENT '城市id',
  `city_name` varchar(50) DEFAULT NULL,
  `adviser_id` int(10) NOT NULL,
  `adviser_name` varchar(50) DEFAULT NULL,
  `accounting` decimal(15,2) NOT NULL COMMENT 'xx',
  `receivable` decimal(15,2) NOT NULL DEFAULT '0.00' COMMENT '当xx',
  `contract_type` tinyint(1) NOT NULL DEFAULT '1' COMMENT '1:xx合同;2:xx合同;3:xx合同',
  PRIMARY KEY (`id`),
  KEY `contract_id` (`contract_id`),
  KEY `date` (`date`),
  KEY `city_id` (`city_id`)
) ENGINE=InnoDB AUTO_INCREMENT=734525 DEFAULT CHARSET=utf8

二、业务问题

*  基本信息，由于合同号太多，所以这边就以一个有重复数据的合同id为例

dba:aif_db> select contract_id,contract_no,receivable,date from crm_report_accounting_income_2015_online where contract_id = 27310;
+-------------+----------------------------+------------+----------+
| contract_id | contract_no                | receivable | date     |
+-------------+----------------------------+------------+----------+
|       27310 | A00-SHEN-05-2018-06-004613 |    2941.18 | 20180628 |
|       27310 | A00-SHEN-05-2018-06-004613 |    5882.36 | 20180629 |
|       27310 | A00-SHEN-05-2018-06-004613 |    8823.54 | 20180630 |
|       27310 | A00-SHEN-05-2018-06-004613 |   11764.72 | 20180701 |
|       27310 | A00-SHEN-05-2018-06-004613 |   14705.90 | 20180702 |
|       27310 | A00-SHEN-05-2018-06-004613 |   17647.08 | 20180703 |
|       27310 | A00-SHEN-05-2018-06-004613 |   20588.26 | 20180704 |
|       27310 | A00-SHEN-05-2018-06-004613 |   23529.44 | 20180705 |
|       27310 | A00-SHEN-05-2018-06-004613 |   26470.62 | 20180706 |
|       27310 | A00-SHEN-05-2018-06-004613 |   29411.80 | 20180707 |
|       27310 | A00-SHEN-05-2018-06-004613 |   32352.98 | 20180708 |
|       27310 | A00-SHEN-05-2018-06-004613 |   35294.16 | 20180709 |
+-------------+----------------------------+------------+----------+
12 rows in set (0.00 sec)



* 查询每个最新合同的信息，由于合同号太多，所以这边就以一个有重复数据的合同id为例

select contract_no, contract_id, city_name, receivable,date from
(select * from crm_report_accounting_income_2015_online  where contract_id = 27310 ORDER BY `date` desc) p GROUP BY contract_id

+----------------------------+-------------+-----------+------------+----------+
| contract_no                | contract_id | city_name | receivable | date     |
+----------------------------+-------------+-----------+------------+----------+
| A00-xxxx-05-2018-06-xxxxxx |       xxxxx | 沈阳      |    2941.18 | 20180628 |
+----------------------------+-------------+-----------+------------+----------+
1 row in set (0.00 sec)

以上看到的写法，是通过子查询写的，5.6查询没问题，5.7就变成了以上的结果，很明显得到的答案不是业务想要的

究其原因还是因为，MySQL5.7 sql mode更加严格了，如果设置sql_mode = ONLY_FULL_GROUP_BY, 那么以上SQL就会报错

因为sql_mode = ONLY_FULL_GROUP_BY 要求符合SQL 92标准，即：select列表里只能出现分组列（即group by后面的列）和聚合函数（max，min等等）

然而为了兼容5.6，我们设置sql_mode='', 所以我们的Group by 在子查询中就跟5.6就不一致了

当然，我们应该避免不标准的SQL写法，这样的问题，我们的解法就是调整业务的SQL语句，改写成SQL 92标准的语法

那么以上SQL语句应该调整为：

select
    contract_no,
    e.contract_id,
    city_name,
    receivable,
    date
from
    crm_report_accounting_income_2015_online e,
    ( select contract_id , max(date) max_date from  crm_report_accounting_income_2015_online where contract_id = 27310 group by contract_id  ) t
where
    e.contract_id = t.contract_id
    and e.date = t.max_date

+----------------------------+-------------+-----------+------------+----------+
| contract_no                | contract_id | city_name | receivable | date     |
+----------------------------+-------------+-----------+------------+----------+
| A00-xxxx-05-2018-06-004613 |       27310 | xxxx      |   35294.16 | 20180709 |
+----------------------------+-------------+-----------+------------+----------+
1 row in set (0.00 sec)

以上都还是需要业务代码修改，这样如果没有提前发现问题，岂不是会导致业务出错了？有没有更好的办法？

MySQL方面其实还是可以配置相关的参数的：

dba:aif_db> set optimizer_switch='derived_merge=off';
Query OK, 0 rows affected (0.00 sec)

dbadmin:aifangcrm_db> select contract_no, contract_id, city_name, receivable,date from
    -> (select * from crm_report_accounting_income_2015_online  where contract_id = 27310 ORDER BY `date` desc) p GROUP BY contract_id
    -> ;
+----------------------------+-------------+-----------+------------+----------+
| contract_no                | contract_id | city_name | receivable | date     |
+----------------------------+-------------+-----------+------------+----------+
| A00-xxxx-05-2018-06-004613 |       27310 | xxxx      |   35294.16 | 20180709 |
+----------------------------+-------------+-----------+------------+----------+
1 row in set (0.00 sec)

三、总结

• SQL语法应该要按照标准的SQL92来写
• 数据库升级到5.7之后，应该提前监控处group by + 子查询的情况，提前告知业务修改业务代码
• 设置参数也能解决问题，但是这个参数毕竟是5.7新增的，如果关闭后，以后会不会导致其他的bug就不知晓了

最后，还是希望能够修改query 语句到标准语法，如果出现业务问题，可以让业务修改参数快速解决问题，然后再修改语句比较与时俱进