以下兩種方式啟動都報錯：啟動失敗：
[root@node03 ~]# service mysqld start
MySQL Daemon failed to start.
Starting mysqld: [FAILED]
[root@node03 ~]# /etc/init.d/mysqld start
MySQL Daemon failed to start.
Starting mysqld: [FAILED]
原因：舊版本的MySQL軟件移除時沒有刪除原來的數據文件，即 /var/lib/mysql/路徑下的內容。
解決方法：刪除該路徑下的所有內容，包括文件和文件夾。
再次啟動，報錯另一個錯誤：
[root@node03 ~]# service mysqld start
Initializing MySQL database: 2019-06-26T16:37:53.977336Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2019-06-26T16:37:54.359914Z 0 [Warning] InnoDB: New log files created, LSN=45790
2019-06-26T16:37:54.431696Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2019-06-26T16:37:54.502670Z 0 [ERROR] unknown variable 'default-character-set=utf8'
2019-06-26T16:37:54.502687Z 0 [ERROR] Aborting
[FAILED]
原因：/etc/my.cnf的配置文件中，我配置了default-character-set，但MySQL5.7貌似不認該參數。
解決方法：把所有的default-character-set配置項注釋掉，或刪除掉。
再次啟動，報第三個錯誤：
[root@node03 ~]# service mysqld start
Initializing MySQL database: 2019-06-26T16:46:34.713514Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2019-06-26T16:46:34.715422Z 0 [ERROR] --initialize specified but the data directory has files in it. Aborting.
2019-06-26T16:46:34.715447Z 0 [ERROR] Aborting
[FAILED]
原因：由於第二次的啟動失敗，在/var/lib/mysql/路徑下殘留了一些文件，由於MySQL初始化時需要該路徑下沒有任何內容，所以啟動失敗。
解決方法：清空該路徑下的所有內容。
再次啟動，成功！
[root@node03 ~]# service mysqld start
Initializing MySQL database: 2019-06-26T16:47:53.295945Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp se er option (see documentation for more details).
2019-06-26T16:47:53.633116Z 0 [Warning] InnoDB: New log files created, LSN=45790
2019-06-26T16:47:53.699952Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2019-06-26T16:47:53.817335Z 0 [Warning] No existing UUID has been found, so we assume that this is the first time that this server has been started. Generating a new UUID: 246937da-9832-11e9-9e76-00505632a066.
2019-06-26T16:47:53.826311Z 0 [Warning] Gtid table is not ready to be used. Table 'mysql.gtid_executed' cannot be opened.
2019-06-26T16:47:53.832203Z 1 [Note] A temporary password is generated for root@localhost: nlaJ#thW=9zQ
[ OK ]
Starting mysqld: [ OK ]