解决方法：
1.查看是不是mysql权限的问题 （默认安装方式为 yum -y install mysql）
转到 /etc/rc.d/init.d/目录下
[root@xxx init.d]# mysqld status
2018-06-11 15:20:47 0 [Warning] TIMESTAMP with implicit DEFAULT value is depreca ted. Please use --explicit_defaults_for_timestamp server option (see documentati on for more details).
2018-06-11 15:20:47 0 [Note] mysqld (mysqld 5.6.40) starting as process 12917 .. .
2018-06-11 15:20:47 12917 [ERROR] Fatal error: Please read "Security" section of the manual to find out how to run mysqld as root!
2018-06-11 15:20:47 12917 [ERROR] Aborting
2018-06-11 15:20:47 12917 [Note] Binlog end
2018-06-11 15:20:47 12917 [Note] mysqld: Shutdown complete
其中的第一个错误是
2018-06-11 15:20:47 12917 [ERROR] Fatal error: Please read "Security" section of the manual to find out how to run mysqld as root!
这是因为MySQL为了安全，不希望root用户直接启动mysql。
所以可以在 /etc/my.cnf里增加 user=mysql 来指定mysql的用户
--------------------------------------------------------------------
[mysqld]
user=mysql
--------------------------------------------------------------------
[root@xxx search]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.6.40 MySQL Community Server (GPL)
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
可以看到mysql启动成功。