title: How to validate that introducing a foreign key constraint won't cause issues
body: <p>I have two tables, table A and table B. Table A has a column named customerId. I'd like to create a foreign key between table B's customerId to table A's customerId. I have a massive database, and I'd like to verify that all of table B's customerIds have a valid link to table A's customerIds before introducing the foreign key. </p>
  
  <p>Is there a SQL query to view any rogue data before introducing the foreign key? </p>