i>
<li>Assertion</li>
</ul></li>
</ul>
<h3 id="null-or-not-null-constraints"><code>null</code> or <code>not null</code> Constraints</h3>
<ul>
<li>A column can allow <code>null</code> values</li>
<li>Or disallow them with <code>not null</code></li>
<li>Default: allow <code>null</code> values</li>
</ul>
<h3 id="primary-key-constraints">Primary Key Constraints</h3>
<ul>
<li>A column or group of columns can be designated as the <code>primary key</code></li>
<li>Primary key columns cannot be <code>null</code>
<u