title: SQL select count from multiple tables
body: <p>I'm a starter at SQL and I have the following tables, <code>ORDER_PRODUCTS</code>, listing the products of an order and <code>EXCHANGE_PRODUCTS</code>, listing products that will be exchanged.</p>
  
  <p>Both have the same fields, and I need to make a selection counting the <code>amount</code> of products in both tables, distinguishing them by the <code>order_id</code>, does anyone knows how I can do this?</p>
  
  <p>ORDER_PRODUCTS</p>
  
  <pre><code>+-----+------------+----------+---------+
  |  id | product_id | order_id | amount  |
  +-----+------------+----------+---------+
  |   1 |          5 |        1 |       2 |
  |   2 |          7 |        1 |       1 |
  |   3 |         13 |        5 |       1 |
  |   4 |         18 |        8 |       3 |
  |   5 |         45 |       11 |       4 |
  +-----+------------+----------+---------+
  </code></pre>
  
  <p>EXCHANGE_PRODUCTS</p>
  
  <pre><code>+-----+------------+----------+---------+
  |  id | product_id | order_id | amount  |
  +-----+------------+----------+---------+
  |   1 |          5 |        1 |       1 |
  |   2 |          7 |        1 |       2 |
  |   3 |         13 |        5 |       1 |
  |   4 |          3 |        8 |       2 |
  |   5 |          2 |       11 |       1 |
  +-----+------------+----------+---------+
  </code></pre>