centos7.8 open mysql3306 port

1. Check centos version    

cat /etc/redhat-release

2. Check the status of the firewall: active (running) is the open state

systemctl status firewalld

3. Add a firewall open port:

firewall-cmd --zone=public --add-port=3306/tcp --permanent

4. After opening the port, the firewall needs to be reloaded:

firewall-cmd --reload

5. Query whether the specified port is open 

firewall-cmd --query-port=3306/tcp

Reference article  linux firewalld firewall status query and open ports