title: Which customer has placed most orders. SQL query
body: <p>I'm trying to query my database for my class to find out which customer has placed the most orders. The table I'm searching is a three attribute table that has the customerID, orderID, and the placedDate. </p>
  
  <p>The query I thought would work is:</p>
  
  <pre><code>select cid from placed order by sum(oid);
  </code></pre>
  
  <p>But I keep getting an error saying cid is "not a single-group group function" the oid is the primary key and is a foreign key that references another table. Is that what the issue is? </p>