title: Mysql Return true if idx exists in another table
body: <p>I'm trying to write a query that returns true if the same idx exists in another table. </p>
  
  <p>Here is what I want to do.</p>
  
  <p>Two Tables:</p>
  
  
  
  <pre><code>User (user_idx, name)
  Group (group_idx, user_idx)
  </code></pre>
  
  <p>Pseudo Query:</p>
  
  
  
  <pre><code>SELECT user_idx, name, (True(1)/False(0) value) as has_joined_group
  FROM User
  WHERE (Check if user_idx exists in Group table where group_idx is 3)
  </code></pre>
  
  <p>Can this be done using Mysql? If yes, how?</p>