title: When to use Null vs. N/A in columns?
body: <p>Suppose you have 2 columns in a table: FixedSalary and HourlyRate. The value for one of these columns is always going to be missing because an employee cannot be paid both a fixed salary and an hourly rate.</p>
  
  <p>Now, suppose I have another table HairColor column. You're updating a row for a patient who is bald. The value will be missing.</p>
  
  <p>For both examples, we can either use a Null value or something like "N/A" or "Not applicable". I was wondering, is there a best (database design) practice regarding these situations, where there is a clearly lack of value, but that lack of value is for a very clear logical reason?</p>