title: SQL Check one list against Another
body: <p>I'd appreciate any pointers on how in SQL to check whether elements in one list also appear in another.</p>
  
  <pre><code>List A = Live Customers in April
  List B = Live Customers in May
  </code></pre>
  
  <p>How can I check which Customers in List A also appear in List B ? to identify those Customers which have been lost </p>
  
  <pre><code>i.e. Customers in A but not in B.
  </code></pre>
  
  <p>Thank you for your help. Gav</p>