title: Multi column primary keys?
body: <p>For example I have 2 tables, <code>Users</code> and <code>UserRelations</code>, and it is a one to many relationship.</p>
  
  <p>For the <code>UserRelations</code> table, I can have an identity column and make it the primary key: </p>
  
  <pre><code>[RelationID] [int] IDENTITY(1,1) NOT NULL,
  [UserID] [int] NOT NULL,
  [TargetID] [int] NOT NULL,
  </code></pre>
  
  <p>Or I can design the table like:</p>
  
  <pre><code>[UserID] [int] NOT NULL,
  [TargetID] [int] NOT NULL,
  </code></pre>
  
  <p>and make <code>UserID</code> + <code>TargetID</code> the primary key.</p>
  
  <p>My question is what are the implications of going with each design, which is better for performance?</p>