下载MySQL的rpm包安装MySQL

cd /usr/local/src

wget https://cdn.mysql.com/Downloads/MySQL-5.7/mysql-community-server-5.7.27-1.el7.x86_64.rpm
wget https://cdn.mysql.com/Downloads/MySQL-5.7/mysql-community-client-5.7.27-1.el7.x86_64.rpm
wget https://cdn.mysql.com/Downloads/MySQL-5.7/mysql-community-libs-5.7.27-1.el7.x86_64.rpm
wget https://cdn.mysql.com/Downloads/MySQL-5.7/mysql-community-common-5.7.27-1.el7.x86_64.rpm

yum remove mariadb-libs
yum install net-tools
yum install *.rpm

systemctl status mysqld.service
systemctl start mysqld.service

# 查看默认密码
grep 'temporary password' /var/log/mysqld.log
liykpntuu9?C

# 登陆后执行命令出现如下提示：
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.

解决办法：使用初始密码登陆后需要修改root密码才行
alter user 'root'@'localhost' identified by '123liykpntuu9?C';
flush privileges;

然后退出重新登陆操作即可

# mysql5.6是密码为空直接进入数据库的，但是mysql5.7就需要初始密码
cat /var/log/mysqld.log | grep password

# 然后执行 mysql -uroot -p ，输入上面的到的密码进入，用该密码登录后，必须马上修改新的密码，不然会报如下错误：
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.

# 如果你想要设置一个简单的测试密码的话，比如设置为123456，会提示这个错误，报错的意思就是你的密码不符合要求
mysql> alter user 'root'@'localhost' identified by '123456';
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

这个其实与validate_password_policy的值有关。

validate_password_policy有以下取值：

默认是1，即MEDIUM，所以刚开始设置的密码必须符合长度，且必须含有数字，小写或大写字母，特殊字符。
有时候，只是为了自己测试，不想密码设置得那么复杂，譬如说，我只想设置root的密码为123456。
必须修改两个全局参数：

首先，修改validate_password_policy参数的值
mysql> set global validate_password_policy=0;
Query OK, 0 rows affected (0.00 sec)

validate_password_length(密码长度)参数默认为8，我们修改为1

mysql> set global validate_password_length=1;
Query OK, 0 rows affected (0.00 sec)

# 完成之后再次执行修改密码语句即可成功
mysql> alter user 'root'@'localhost' identified by '123456';
Query OK, 0 rows affected (0.00 sec)