title: Meaning of Primary Key to Microsoft SQL Server 2008
body: <p>What meaning does the concept of a primary key have to the database engine of SQL Server? I don't mean the clustered/nonclustered index created on the "ID" column, i mean the constraint object "primary key". Does it matter if it exists or not?</p>
  
  <p>Alternatives:</p>
  
  <ol>
  <li>alter table add primary key clustered</li>
  <li>alter table create clustered index</li>
  </ol>
  
  <p>Does it make a difference?</p>