title: How to return values where one value is in both tables but another is in only one?
body: <p>I have two tables, which look roughly like this:</p>
  <pre><code>Table1:
  
   CUSTOMER OFFER SALES
   -------- ----- -----
   A          111     1
   A          222     1
   B          111     1
   B          222     1
   C          111     1
  
  
  Table2:
  
   CUSTOMER OFFER
   -------- -----
   A          111
   A          222
   B          111
  </code></pre>
  <p>I want to query these tables to return only the customers who are in both tables, but also to only return the rows where that combination of customer &amp; offer does not exist in table2.</p>
  <p>In other words, the results should look like this:</p>
  <pre><code> CUSTOMER OFFER SALES
   -------- ----- -----
   B          222     1
  </code></pre>
  <p>Does anyone know how best to do this please?</p>