other table we refer to
<ul>
<li>this is often called a <strong>referential integrity constraint</strong>
<ul>
<li>this usually corresponds to a <em>many-exactly-one</em> relationship</li>
</ul></li>
<li>exception: if we allow <code>null</code> values for the foreign key column(s), then <code>null</code> values don’t have to exist in the other table
<ul>
<li>and <code>null</code> values usually won’t exist because the column(s) the FK refers to will usually be primary key columns</li>
<li>this usually c