that shows the tables and their relationships 
is called a <b>data model</b>.<br />
<br />
Data modeling is a relatively sophisticated skill and we will only introduce
the most basic concepts of relational data modeling in this section. For more
detail on data modeling you can start with:<br />
<br />
<tt>http://en.wikipedia.org/wiki/Relational_model</tt><br />
<br />
Let's say for our Twitter spider application, instead of just 
counting a person's friends, we wanted to keep a list of 
all of the incoming 