title: SELECT with INNER JOIN (or other) setting value for result that is not in the second table
body: <p>I have no idea how to do this with SQL (Firebird 2.5).</p>
  
  <p>I have two tables. In the first table I have a list with the product information and in the second table I have the orders that use, by relationship with primary keys, the products of the first table.</p>
  
  <p>I need to select all the items in the product table and take the sum of the entire quantity of products used in the order table. If the product was not used in any order, I still need the product listed, with a sum equal to zero.</p>