I guuss Equation 3.1 should read:

**Equation 3.1**

$\begin{array}{c|ccc} Employee & FName & WorksIn & Mngr \\\\ \hline 1 & Alan & 101 & 2 \\\\ 2 & Ruth & 101 & 2 \\\\ 3 & Kris & 102 & 3 \end{array}$

$\begin{array}{c|cc} Department & DName & Secr& \\\\ \hline 101 & Sales & 1 \\\\ 102 & IT & 3 \end{array}$

In which case, the number of non-ID columns is 5, the same as the number of arrows in Equation 3.2. This is not a coincidence, since Equation 3.1 is an instantiation of Equation 3.2. In particular, each non-ID column represents the image of the ID column under a function, which is the instantiation of one of the arrows in Equation 3.2.