How do i manipulate third uri segment ?

[eluser]dhaulagiri[/eluser]
I thought 3rd segment of the URI directly corresponds to database id, as i assume, what it happens in other application.

for example I have 25 rows of data in a table, I want to point that database id in a url and fetch all the data on the corresponding fields of the 'id'.

how do i assign id in such scene?
I have seen example somewhere $this->uri->segment(3) = $id . How will this work with my database ? Sorry I am realtively new.