mysql操作命令master-to-master
1.select语句中把两个字符串字段合并成一个字段：
select concat(a, b) as c from tables
2.按日期分组查询
select DATE_FORMAT( created_at, '%Y-%m-%d %H' ) as t from tables group by t
3.mysql创建用户/添加远程/修改用户密码
先进入mysql命令：mysql -uroot -proot
->create user username@'192.168.39.%' IDENTIFIED BY 'password';
配置远程
->GRANT REPLICATION SLAVE,REPLICATION CLIENT ON *.* TO username@'192.168.39.%' IDENTIFIED BY 'password';
修改密码
->update mysql.user set password=PASSWORD('xxx') where user='username';
->flush privileges;
4.master-to-master
先配置远程：
GRANT REPLICATION SLAVE,REPLICATION CLIENT ON *.* TO username@'192.168.39.%' IDENTIFIED BY 'password';
建立master:
->slave stop;
->change master to master_host='192.168.0.2',
->master_user='backup',
->master_password='back',
->master_log_file='mysql-bin.000001',
->master_log_pos=106;
->slave start;
->show slave status\G
4.查看server_id，注意master-to-master时，两个服务器的server_id不能一样，需要修改。
->show variables like 'server_id';
修改server_id
->set global server_id=10;
5.查看auto_increment_increment与auto_increment_offset值:
->SHOW VARIABLES LIKE 'auto_inc%';
6.两个服务器master-to-master，必须设置auto_increment_offset值为不同，如下：
第一台：
auto_increment_increment =2 #为开始值
auto_increment_offset = 1
第二台：
auto_increment_increment =2 #为开始值
auto_increment_offset = 2
7.mysql远程链接权限有以下14个：
select,insert,update,delete,create,drop,index,alter,grant,references,reload,shutdown,process,file