title: SQL Sub queries in check constraint
body: <p>Can I make SQL sub queries in Check constraint ?</p>
  
  <p>I've a <code>post</code> table with columns <code>id, owner</code><br />
  I've another table <code>action</code> with columns <code>user_id, post_id</code><br />
  Table <code>user</code> with columns <code>id</code></p>
  
  <p><code>post_id -&gt; post.id</code> and <code>user_id -&gt; user.id</code> also <code>post.owner -&gt; user.id</code></p>
  
  <p>Now I want to constraint <code>post(post_id).id != user_id</code> on table <code>action</code></p>
  
  <p>How is that possible ?</p>