title: find records with an exact number of children
body: <p>Database structure:</p>
  
  <p>Restaurant</p>
  
  <ul>
  <li>rID (Primary)</li>
  </ul>
  
  <p>Retsau_Attribute</p>
  
  <ul>
  <li>id (primary)</li>
  <li>rID (foreign)</li>
  <li>aID (foreign)</li>
  </ul>
  
  <p>Attributes</p>
  
  <ul>
  <li>aID (Primary)</li>
  </ul>
  
  <p>I want to extract all the restaurant IDs that have all the attributes specified in the query.</p>
  
  <p>I tried to use the IN operator, but it returns all restaurant IDs that have some or all of the attributes.</p>
  
  <p>I used this query:</p>
  
  <pre><code>select rID from Retsau_Attribute where aID IN (a1, a2, a3);
  </code></pre>
  
  <p>I need only those restaurant IDs that have all the attributes specified (a1, a2, a3).</p>