title: Best way to store whether data exists in a relational database?
body: <p>If we ask the following question of an individual: "Does that person have children?" There are three possible answers: "Yes", "No", "Unknown". With a table design that stores the person as a record in a <code>people</code> table, and a child as a record in a <code>children</code> table with a foreign key to the <code>people</code> table, we cannot distinguish between "No" and "Unknown". Both answers imply no rows in the <code>children</code> table.</p>
  
  <p>What is the best way to store the answer to that question that can distinguish between the three states? My current thought is storing that information as a separate field on the <code>people</code> table, but that opens up data integrity issues.</p>