title: Filter rows in SQL by sub-query/ joins
body: <p>Here is a table of user, product and month.</p>
  <pre><code>CREATE TABLE USERS (user VARCHAR(1),product VARCHAR(1),month INT);
  INSERT INTO USERS (user,product,month) VALUES 
  (&quot;A&quot;,&quot;X&quot;,9),(&quot;A&quot;,&quot;X&quot;,10),(&quot;A&quot;,&quot;X&quot;,10),(&quot;A&quot;,&quot;Y&quot;,11),
  (&quot;B&quot;,&quot;Y&quot;,12),(&quot;B&quot;,&quot;X&quot;,10),(&quot;C&quot;,&quot;X&quot;,10),(&quot;D&quot;,&quot;Z&quot;,11),(&quot;D&quot;,&quot;X&quot;,9);
  </code></pre>
  <p>How do I count the number of users of a certain product only (possibly also in a specific month)?</p>