title: Unique Constraint column can only contain one NULL value
body: <p>A Unique Constraint can be created upon a column that can contain NULLs. However, at most, only a single row may ever contain a NULL in that column.</p>
  
  <p>I do not understand why this is the case since, by definition, a NULL is not equal to another NULL (since NULL is really an unknown value and one unknown value does not equal another unknown value).</p>
  
  <p>My questions:
  1. Why is this so?
  2. Is this specific to MsSQL?</p>
  
  <p>I have a hunch that it is because a Unique Constraint can act as a reference field for a Foreign Key and that the FK would otherwise not know which record in the reference table to which it was refering if more than one record with NULL existed. But, it is just a hunch.</p>
  
  <p>(Yes, I understand that UCs can be across multiple columns, but that doesn't change the question; rather it just complicates it a bit.)</p>