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一、先排序后分组问题：
数据准备：
[work@xxx scripts]$ mysql -hyyy -P3306 -uroot -proot WARNING: DATABASE IS WRITEABE MAY BE MASTER, BE CAREFUL Warning: Using a password on the command line interface can be insecure. Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 111044039 Server version: 5.5.31-log MySQL Community Server (GPL) Copyright (c) 2009-2015 Percona LLC and/or its affiliates Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or 'h' for help. Type 'c' to clear the current input statement. Mon Feb 22 18:40:09 2021 root@xxx((none)) > use apple Database changed Mon Feb 22 18:40:16 2021 root@xxx(apple) > show create table t_userG *************************** 1. row *************************** Table: t_user Create Table: CREATE TABLE `t_user` ( `id` bigint(20) NOT NULL AUTO_INCREMENT, `name` varchar(255) DEFAULT NULL, `phone` varchar(255) DEFAULT NULL, `gender` varchar(255) DEFAULT NULL, `type` varchar(255) DEFAULT NULL, `birth` datetime DEFAULT NULL, `is_delete` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8 1 row in set (0.00 sec) Mon Feb 22 18:40:46 2021 ###################### ---插入数据
INSERT INTO `t_user` VALUES ('1', 'James', '0594-5397864', '0', '3', '2016-01-30 19:01:09', '1'); INSERT INTO `t_user` VALUES ('2', 'Hayes', '0594-5392419', '1', '4', '2015-12-24 11:12:27', '1'); INSERT INTO `t_user` VALUES ('3', 'Diana', '0594-5393520', '1', '5', '2016-03-21 13:03:50', '0'); INSERT INTO `t_user` VALUES ('4', 'Rajah', '0594-5399812', '1', '4', '2015-11-26 02:11:35', '0'); INSERT INTO `t_user` VALUES ('5', 'Daria', '0594-5397571', '0', '4', '2016-01-18 11:01:11', '1'); INSERT INTO `t_user` VALUES ('6', 'Lee', '0594-5394539', '1', '1', '2015-10-23 08:10:23', '1'); INSERT INTO `t_user` VALUES ('7', 'Cameran', '0594-5392867', '0', '4', '2016-11-16 12:11:08', '0'); INSERT INTO `t_user` VALUES ('8', 'Wylie', '0594-5395349', '0', '5', '2017-07-06 04:07:27', '0'); INSERT INTO `t_user` VALUES ('9', 'Bertha', '0594-5395287', '1', '1', '2017-02-08 12:02:45', '1'); INSERT INTO `t_user` VALUES ('10', 'Fletcher', '0594-5399246', '0', '4', '2015-09-03 20:09:33', '0'); INSERT INTO `t_user` VALUES ('11', 'Conan', '0594-5391546', '1', '5', '2017-05-15 09:05:23', '0'); INSERT INTO `t_user` VALUES ('12', 'Raymond', '0594-5399666', '0', '3', '2015-10-20 05:10:05', '1'); INSERT INTO `t_user` VALUES ('13', 'Noel', '0594-5397392', '1', '4', '2017-05-26 03:05:56', '0'); INSERT INTO `t_user` VALUES ('14', 'Miriam', '0594-5399081', '0', '2', '2016-05-21 02:05:09', '0'); INSERT INTO `t_user` VALUES ('15', 'Maya', '0594-5397242', '0', '3', '2016-10-24 02:10:50', '1'); INSERT INTO `t_user` VALUES ('16', 'Winifred', '0594-5395142', '1', '1', '2017-03-15 02:03:43', '0'); INSERT INTO `t_user` VALUES ('17', 'Elaine', '0594-5398478', '1', '3', '2017-03-08 15:03:03', '1'); INSERT INTO `t_user` VALUES ('18', 'Robert', '0594-5397830', '0', '5', '2016-02-10 22:02:06', '0'); INSERT INTO `t_user` VALUES ('19', 'Patrick', '0594-5396516', '0', '4', '2015-09-10 07:09:51', '0'); INSERT INTO `t_user` VALUES ('20', 'Darrel', '0594-5397417', '0', '1', '2016-03-11 11:03:36', '0'); INSERT INTO `t_user` VALUES ('21', 'Salvador', '0594-5399732', '1', '3', '2016-01-01 15:01:21', '0'); INSERT INTO `t_user` VALUES ('22', 'Brandon', '0594-5396204', '1', '4', '2016-05-12 06:05:40', '1'); INSERT INTO `t_user` VALUES ('23', 'Dorothy', '0594-5396783', '0', '1', '2016-12-12 10:12:59', '1'); INSERT INTO `t_user` VALUES ('24', 'Kevyn', '0594-5398240', '0', '2', '2016-02-07 04:02:14', '1'); INSERT INTO `t_user` VALUES ('25', 'Brody', '0594-5398774', '1', '1', '2016-12-11 20:12:36', '0');
mysql5.5上执行：
root@xxx(apple) > select t1.* from t_user t1; +----+----------+--------------+--------+------+---------------------+-----------+ | id | name | phone | gender | type | birth | is_delete | +----+----------+--------------+--------+------+---------------------+-----------+ | 1 | James | 0594-5397864 | 0 | 3 | 2016-01-30 19:01:09 | 1 | | 2 | Hayes | 0594-5392419 | 1 | 4 | 2015-12-24 11:12:27 | 1 | | 3 | Diana | 0594-5393520 | 1 | 5 | 2016-03-21 13:03:50 | 0 | | 4 | Rajah | 0594-5399812 | 1 | 4 | 2015-11-26 02:11:35 | 0 | | 5 | Daria | 0594-5397571 | 0 | 4 | 2016-01-18 11:01:11 | 1 | | 6 | Lee | 0594-5394539 | 1 | 1 | 2015-10-23 08:10:23 | 1 | | 7 | Cameran | 0594-5392867 | 0 | 4 | 2016-11-16 12:11:08 | 0 | | 8 | Wylie | 0594-5395349 | 0 | 5 | 2017-07-06 04:07:27 | 0 | | 9 | Bertha | 0594-5395287 | 1 | 1 | 2017-02-08 12:02:45 | 1 | | 10 | Fletcher | 0594-5399246 | 0 | 4 | 2015-09-03 20:09:33 | 0 | | 11 | Conan | 0594-5391546 | 1 | 5 | 2017-05-15 09:05:23 | 0 | | 12 | Raymond | 0594-5399666 | 0 | 3 | 2015-10-20 05:10:05 | 1 | | 13 | Noel | 0594-5397392 | 1 | 4 | 2017-05-26 03:05:56 | 0 | | 14 | Miriam | 0594-5399081 | 0 | 2 | 2016-05-21 02:05:09 | 0 | | 15 | Maya | 0594-5397242 | 0 | 3 | 2016-10-24 02:10:50 | 1 | | 16 | Winifred | 0594-5395142 | 1 | 1 | 2017-03-15 02:03:43 | 0 | | 17 | Elaine | 0594-5398478 | 1 | 3 | 2017-03-08 15:03:03 | 1 | | 18 | Robert | 0594-5397830 | 0 | 5 | 2016-02-10 22:02:06 | 0 | | 19 | Patrick | 0594-5396516 | 0 | 4 | 2015-09-10 07:09:51 | 0 | | 20 | Darrel | 0594-5397417 | 0 | 1 | 2016-03-11 11:03:36 | 0 | | 21 | Salvador | 0594-5399732 | 1 | 3 | 2016-01-01 15:01:21 | 0 | | 22 | Brandon | 0594-5396204 | 1 | 4 | 2016-05-12 06:05:40 | 1 | | 23 | Dorothy | 0594-5396783 | 0 | 1 | 2016-12-12 10:12:59 | 1 | | 24 | Kevyn | 0594-5398240 | 0 | 2 | 2016-02-07 04:02:14 | 1 | | 25 | Brody | 0594-5398774 | 1 | 1 | 2016-12-11 20:12:36 | 0 | +----+----------+--------------+--------+------+---------------------+-----------+ 25 rows in set (0.00 sec) Mon Feb 22 18:41:00 2021 root@xxx(apple) > select t2.* from (select t1.* from t_user t1 order by t1.id desc) as t2 group by t2.type; +----+----------+--------------+--------+------+---------------------+-----------+ | id | name | phone | gender | type | birth | is_delete | +----+----------+--------------+--------+------+---------------------+-----------+ | 25 | Brody | 0594-5398774 | 1 | 1 | 2016-12-11 20:12:36 | 0 | | 24 | Kevyn | 0594-5398240 | 0 | 2 | 2016-02-07 04:02:14 | 1 | | 21 | Salvador | 0594-5399732 | 1 | 3 | 2016-01-01 15:01:21 | 0 | | 22 | Brandon | 0594-5396204 | 1 | 4 | 2016-05-12 06:05:40 | 1 | | 18 | Robert | 0594-5397830 | 0 | 5 | 2016-02-10 22:02:06 | 0 | +----+----------+--------------+--------+------+---------------------+-----------+ 5 rows in set (0.00 sec) Mon Feb 22 18:41:11 2021 root@xxx(dba_metadata) >
mysql5.7上执行：
[work@zzz scripts]$ mysql -hzzz -P3306 -uroot -proot Warning: Using a password on the command line interface can be insecure. Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 5048729 Server version: 5.7.25-28-log MySQL Community Server (GPL) Copyright (c) 2009-2015 Percona LLC and/or its affiliates Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or 'h' for help. Type 'c' to clear the current input statement. Mon Feb 22 18:41:18 2021 root@zzz ((none)) > use apple Database changed Mon Feb 22 18:41:21 2021 root@zzzj (apple) > select t2.* from (select t1.* from t_user t1 order by t1.id desc) as t2 group by t2.type; +----+--------+--------------+--------+------+---------------------+-----------+ | id | name | phone | gender | type | birth | is_delete | +----+--------+--------------+--------+------+---------------------+-----------+ | 6 | Lee | 0594-5394539 | 1 | 1 | 2015-10-23 08:10:23 | 1 | | 14 | Miriam | 0594-5399081 | 0 | 2 | 2016-05-21 02:05:09 | 0 | | 1 | James | 0594-5397864 | 0 | 3 | 2016-01-30 19:01:09 | 1 | | 2 | Hayes | 0594-5392419 | 1 | 4 | 2015-12-24 11:12:27 | 1 | | 3 | Diana | 0594-5393520 | 1 | 5 | 2016-03-21 13:03:50 | 0 | +----+--------+--------------+--------+------+---------------------+-----------+ 5 rows in set (0.00 sec) Mon Feb 22 18:41:29 2021
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解决办法：给排序后面加上limit：
root@zzz (apple) > select t2.* from (select t1.* from t_user t1 order by t1.id desc limit 1000000) as t2 group by t2.type; +----+----------+--------------+--------+------+---------------------+-----------+ | id | name | phone | gender | type | birth | is_delete | +----+----------+--------------+--------+------+---------------------+-----------+ | 25 | Brody | 0594-5398774 | 1 | 1 | 2016-12-11 20:12:36 | 0 | | 24 | Kevyn | 0594-5398240 | 0 | 2 | 2016-02-07 04:02:14 | 1 | | 21 | Salvador | 0594-5399732 | 1 | 3 | 2016-01-01 15:01:21 | 0 | | 22 | Brandon | 0594-5396204 | 1 | 4 | 2016-05-12 06:05:40 | 1 | | 18 | Robert | 0594-5397830 | 0 | 5 | 2016-02-10 22:02:06 | 0 | +----+----------+--------------+--------+------+---------------------+-----------+ 5 rows in set (0.00 sec) Mon Feb 22 18:41:41 2021 root@zzz (apple) >
原因：
如果不加limit，系统会把order by优化掉，即mysql5.7中order by不起作用，
在mysql5.7手册的8.2.2.1中有解释：
子查询的优化是使用半连接的策略完成的(The optimizer uses semi-join strategies to improve subquery execution)使用半连接进行优化，子查询语句必须满足一些标准(In MySQL, a subquery must satisfy these criteria to be handled as a semi-join)。其中一个标准是:必须不是一个包含了limit和order by的语句(It must not have ORDER BY with LIMIT.)
二、sql_mode问题：
MySQL 5.6中，其默认值为"NO_ENGINE_SU BSTITUTION"，可理解为非严格模式，譬如，对自增主键插入空字符串''，虽然提示warning，但并不影响自增主键的生成。但在MySQL 5.7中，其就调整为了严格模式，对于上面这个，其不会提示warning，而是直接报错。
但是这个配置可以通过修改配置即可解决，我们线上升级前后都是为空，因此没啥问题。
如果MySQL5.7中的sql_mode配置了ONLY_FULL_GROUP_BY，那么就要求select列表里只能出现分组列（即group by后面的列）和聚合函数（sum，avg，max等），这也是SQL92的标准。
myql的group by语法为：
select 选取分组中的列 + 聚合函数 from 表名称 group by 分组的列
从语法格式来看，是现有分组，在确定检索的列，检索的列只能是参加分组了的列。
所以问题中，group by 后的a,b,c是先确定的，而select 后面的字段是可变的
正确的语句：
select a,b,c from table_name group by a,b,c,d; select a,b from table_name group by a,b,c; select a,max(a) from table_name group by a,b,c;
以下是错误的：
select a,b,c from table_name group by a,b; select a,b,c from table_name group by a;
三、max_allowed_packet问题：
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