Give one example of a reaction in which two compounds combine to form a single compound.

What is the difference between the following two types of reaction?

$$\begin{gathered} {\mathrm{AgNO}}_3 + \mathrm{HCl}\to \mathrm{AgCl} + {\mathrm{HNO}}_3 \\ \mathrm{Mg} + 2\mathrm{HCl}\to {\mathrm{MgCl}}_2 + {\mathrm{H}}_2 \end{gathered}$$