Installing mysql database server is always necessary if you are running a database driven ecommerce site. Remember running mysql server to a fair extend requires atleast 256mb of RAM in your server. So unless you are running database driven sites you dont absolutely need mysql. The following commands will install mysql 5 server and mysql 5 client.
apt-get install mysql-server mysql-client php5-mysql
Note: If you have already installed php4, you should make a slight change like this.
apt-get install mysql-server mysql-client php4-mysql
The configuration file of mysql is located at: /etc/mysql/my.cnf
Creating users to use MySQL and Changing Root Password
By default mysql creates user as root and runs with no passport. You might need to change the root password.
To change Root Password
mysql> USE mysql;
mysql> UPDATE user SET Password=PASSWORD(’new-password’) WHERE user=’root’;
mysql> FLUSH PRIVILEGES;
To Create User
You must never use root password, so you might need to create a user to connect to mysql database for a PHP script. Alternatively you can add users to mysql database by using a control panel like webmin or phpMyAdmin to easily create or assign database permission to users.