Downgrade MySql from your CentOS installation

Sometimes old version of MySQL Server is required for smooth operation of old application. For that first clean up your existing MySQL installation.   

Step 1. Stop the MySQL service

[root@localhost ~]# sudo systemctl stop mysqld.service 

Step 2. List all installed MySQL packages on your system

[root@localhost ~]# rpm -qa | grep mysql

This command will list all installed MySQL packages on your system. E.g. mysql-libs-8.0.15-3.el7._5.x86_64

Step 3. Remove the installed package one by one:

# yum remove < name displayed in Step 2 >

Eg.

[root@localhost ~]# yum remove mysql-libs-8.0.15-3.el7._5.x86_64

Step 4. Enable required version for installation

Now you can install any version of MySQL Server on CentOS. In this case MySQL 5.5 . For that, Enable the MySQL 5.5 Community version repository using below yum install command.

root@localhost ~]# yum install http://repo.mysql.com/yum/mysql-5.5-community/el/7/x86_64/mysql-community-release-el7-5.noarch.rpm

Step 5: Install MySQL

Once repo is enabled, run yum install -y mysql-community-server to install MySQL Server as shown below.

root@localhost ~]# yum install -y mysql-community-server

Step 6: Check if it is installed properly using rpm tool

After successful installation of all the MySQL packages you can verify the installation by querying the package details from RPM DB using rpm -qa | grep -i mysql-community command as shown below. It will show all the installed packages in the RPM Database.

root@localhost ~]# rpm -qa | grep -i mysql-community

Step 7: Start MySQL Service 

Now start the MySQL server using the command

root@localhost ~]# systemctl start  mysqld.service

Step 8: Setting Up MySQL Server Using mysql_secure_installation

This is very important step where you need to configure the Server credentials using mysql_secure_installation utility before login to the Database and set the other important options as well to secure the database.

root@localhost ~]# sudo mysql_secure_installation

After performing above step you need to again restart the server either using systemctl restart mysqld or service mysqld restart command as shown below.

[root@localhost ~]# systemctl restart mysqld

Step 9: Login to MySQL

After setting the root user credentials you can try to login to the MySQL Server using command

[root@localhost ~]# mysql -u root -p 

You can use -u option to specify the User Name and -p option to specify the User Password. This is the same password which you have set in Step 8.