title: How to create a DataRelation from one column in a table to two columns in another table
body: <p>I have a DataTable called <code>test_results</code> with the following columns:</p>
  
  <ol>
  <li>test_results_id</li>
  <li>student_id</li>
  <li>subject</li>
  <li>score</li>
  </ol>
  
  <p>and a DataTable called <code>students</code> with the column:</p>
  
  <ul>
  <li>student_id</li>
  </ul>
  
  <p>I would like to create a DataRelation between the tables so that I can get all <code>test_results</code> rows where </p>
  
  <ul>
  <li>subject = 'math'</li>
  <li>student_id = x</li>
  </ul>
  
  <p>where x is a student_id from <code>students</code></p>
  
  <p>I want to do this so that I can quickly loop through students and find their math results:</p>
  
  <pre class="lang-cs prettyprint-override"><code>foreach (DataRow[] student in students){
      DataRow[] mathResults = student.GetChildRows( "relation_student_math_results" );
      foreach (DataRow[] mathResult in mathResults ){
          // do something
      }
  }
  </code></pre>