This is not entirely realted to ruby but if you ever program with ruby you will find yourself using pg and mysql gems and the issue is how exactly do I install them. This mini-post (rather tumble) is just to make a note of this. Run the following in Teerminal.

PostgreSql

  export PATH=/usr/local/pgsql/bin:${PATH}
  gem install pg

MySql

  export PATH=/usr/local/mysql/bin:${PATH}
  gem install mysql

Also, please note that you should have postgres and mysql installed on your machine.