title: What's wrong with nullable columns in composite primary keys?
body: <p>ORACLE does not permit NULL values in any of the columns that comprise a primary key. It appears that the same is true of most other "enterprise-level" systems.</p>
  
  <p>At the same time, most systems also allow <em>unique</em> contraints on nullable columns.</p>
  
  <p>Why is it that unique constraints can have NULLs but primary keys can not? Is there a fundamental logical reason for this, or is this more of a technical limitation?</p>