title: Select null in a linq query if the value is in a previous record
body: <p>Consider the following query:</p>
  
  <pre><code>var ds = (from t1 in list1
            from t2 in list2
            select new {t1.Name, t2.Name}
           ).ToList();
  </code></pre>
  
  <p>This returns something like: (rough representation)</p>
  
  <pre><code>Name1, InnerName1
  Name1, InnerName2
  Name2, InnerName1
  Name2, InnerName2
  </code></pre>
  
  <p>What I would like to get is:</p>
  
  <pre><code>Name1, InnerName1
  Null, InnerName2
  Name2, InnerName1
  Null, InnerName2.
  </code></pre>
  
  <p>Meaning, <strong>if I have already t1.Name in the list, I'd like null or empty string on the rest of t1's results</strong>.</p>
  
  <p>I already know that I could loop through the results, but I'm using this as a datasource and would like to do something set based.</p>
  
  <p>Is there a way to accomplish this in a single query?</p>