title: Unused columns in SQL subquery
body: <p>I searched the web, but was not sure how to phrase the question.</p>
  <p>Is there performance difference between:</p>
  <pre><code>SELECT t.a FROM (SELECT a, b, c FROM tbl) as t
  </code></pre>
  <p>and</p>
  <pre><code>SELECT t.a FROM 
  (SELECT a FROM tbl) as t
  </code></pre>
  <p>The extra (unused) columns are just plain fields from tbl and are not calculated.</p>
  <p>Also, I am using SQL Server, so if there is a difference, is it the same for every RDBMS?</p>