Mkmf not found

Today, on Ubuntu I tried to manually install mysql. My first problem was this error:

<geshi lang="bash">

sudo ruby extconf.rb

extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)

       from extconf.rb:1:in `<main>'

</geshi>

This was fixed with:

<geshi>

sudo aptitude install ruby1.9-dev

</geshi>