以下两种方式启动都报错：启动失败：
[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：旧版本的MySQL软件移除时没有删除原来的数据文件，即 /var/lib/mysql/路径下的内容。
解决方法：删除该路径下的所有内容，包括文件和文件夹。
再次启动，报错另一个错误：
[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貌似不认该参数。
解决方法：把所有的default-character-set配置项注释掉，或删除掉。
再次启动，报第三个错误：
[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：由于第二次的启动失败，在/var/lib/mysql/路径下残留了一些文件，由于MySQL初始化时需要该路径下没有任何内容，所以启动失败。
解决方法：清空该路径下的所有内容。
再次启动，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 ]