问题：

启动

systemctl start mysqld.service
Job for mysqld.service failed because the control process exited with error code.
See "systemctl status mysqld.service" and "journalctl -xe" for details.

查看状态：

systemctl status mysqld.service
   mysqld.service - MySQL 8.0 database server
   Loaded: loaded (/usr/lib/systemd/system/mysqld.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Mon 2020-04-20 06:37:28 EDT; 11s ago
  Process: 17057 ExecStopPost=/usr/libexec/mysql-wait-stop (code=exited, status=0/SUCCESS)
  Process: 17042 ExecStart=/usr/libexec/mysqld --basedir=/usr (code=exited, status=1/FAILURE)
  Process: 17005 ExecStartPre=/usr/libexec/mysql-prepare-db-dir mysqld.service (code=exited, status=0/SUCCESS)
  Process: 16980 ExecStartPre=/usr/libexec/mysql-check-socket (code=exited, status=0/SUCCESS)
 Main PID: 17042 (code=exited, status=1/FAILURE)
   Status: "Data Dictionary upgrade from MySQL 5.7 in progress"
    Error: 2 (No such file or directory)

Apr 20 06:37:26 lsf1p13 systemd[1]: Starting MySQL 8.0 database server...
Apr 20 06:37:27 lsf1p13 systemd[1]: mysqld.service: Main process exited, code=exited, status=1/FAILURE
Apr 20 06:37:28 lsf1p13 systemd[1]: mysqld.service: Failed with result 'exit-code'.
Apr 20 06:37:28 lsf1p13 systemd[1]: Failed to start MySQL 8.0 database server.

解决办法：

如果是root 可以省略1，2 两步

1. 检查权限： ls -l /var/lib/ | grep mysql
drwxr-xr-x   5 mysql          mysql           270 Apr 20 06:38 mysql
drwxr-x---   2 mysql          mysql             6 Aug  7  2019 mysql-files
drwx------   2 mysql          mysql             6 Aug  7  2019 mysql-keyring

2. 如果没有权限赋权限 ，hmod -R 777 /var/lib/mysql

3.  删除mysql  下的内容，肯能里面的内容有冲突引起的，rm -rf /var/lib/mysql/*

4. 启动mysql, systemctl start mysqld.service

5. 查看状态：

systemctl statys mysqld.service
Unknown operation statys.
[root@lsf1p13 playbook]# systemctl status mysqld.service
   mysqld.service - MySQL 8.0 database server
   Loaded: loaded (/usr/lib/systemd/system/mysqld.service; disabled; vendor preset: disabled)
   Active: active (running) since Mon 2020-04-20 06:47:12 EDT; 22s ago
  Process: 17278 ExecStopPost=/usr/libexec/mysql-wait-stop (code=exited, status=0/SUCCESS)
  Process: 17600 ExecStartPost=/usr/libexec/mysql-check-upgrade (code=exited, status=0/SUCCESS)
  Process: 17472 ExecStartPre=/usr/libexec/mysql-prepare-db-dir mysqld.service (code=exited, status=0/SUCCESS)
  Process: 17447 ExecStartPre=/usr/libexec/mysql-check-socket (code=exited, status=0/SUCCESS)
 Main PID: 17558 (mysqld)
   Status: "Server is operational"
    Tasks: 39
   Memory: 593.1M
   CGroup: /system.slice/mysqld.service
           └─17558 /usr/libexec/mysqld --basedir=/usr

Apr 20 06:47:03 lsf1p13 systemd[1]: Starting MySQL 8.0 database server...
Apr 20 06:47:04 lsf1p13 mysql-prepare-db-dir[17472]: Initializing MySQL database
Apr 20 06:47:12 lsf1p13 systemd[1]: Started MySQL 8.0 database server.

总结：root登录情况下，rm -rf /var/lib/mysql/*，然后重新启动 systemctl status mysqld.service