title: Composite primary key and additional indexes
body: <p>In SQL Server 2005, I have a table with two columns: <code>parent_id (int)</code> and <code>child id (int)</code>. I want make a composite key of them, because I only want exactly one instance per possible combination in the table.</p>
  
  <p>The most search operations will be done on the <code>parent_id</code> field, some on the <code>child_id</code> and only sporadic ones on both fields together. </p>
  
  <p>I have planned to make an index on the <code>parent_id</code> field and maybe also one on the <code>child_id</code> field. Is this meaningful or is SQL Server 2005 capable of using the clustered composite primary key for indexed lookups on only one column (mostly the <code>parent_id</code>) and therefore the index is not necessary/dispensable?</p>