title: oracle intersection with or without distinct
body: <p>I have following query:</p>
  
  <pre><code>select id from t1
  intersect
  select id from t2
  intersect
  select id from t3
  </code></pre>
  
  <p>id could be non unique in some tables, so I need to use distinct.</p>
  
  <p>In general what is better:</p>
  
  <pre><code>select distinct id from (
  select id from t1
  intersect
  select id from t2
  intersect
  select id from t3)
  </code></pre>
  
  <p>or</p>
  
  <pre><code>select distinct id from t1
  intersect
  select id from t2 -- here id is unique
  intersect
  select distinct id from t3
  </code></pre>