title: Column <name> is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause
body: <p>I am fairly new to SQL and have a simple question regarding the error message I received (Title of this question).  I'm trying to find the total value of the items in inventory (itemQuantity * itemCost), the average item cost, and the potential lost revenue (itemQuantity * itemMktValue)</p>
  
  <pre><code>SELECT
      (itemQuantity * itemCost) 'Total',
      AVG (itemCost) 'AvgerageItemCost',
      (itemQuantity * itemMktValue) 'LossedRevenue'
  FROM
      Item;
  </code></pre>
  
  <p>If anyone could help me out it would be much appreciated.</p>