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.