title: Is it possible to set the constraint unique = True except for empty string into SQLAlchemy?
body: <p>I use the following column in my table:</p>
  
  <pre><code>email = Column(String(60), unique=True)
  </code></pre>
  
  <p>However, I need the possibility to save empty strings in this column. Namely, I need something like this:</p>
  
  <pre><code>email = Column(String(60), unique=True, blank=True)
  </code></pre>
  
  <p>but it doesn't work. Is it possible to implement this within SQLAlchemy? What is correspondent SQL (Postgresql dialect)?</p>