title: Mysql unique constraint allowing single row for a combination
body: <p>Is it possible to have a unique constraint such that one particular column has a value only once?</p>
  
  <p>For instance</p>
  
  <pre><code>-----------------------    
  name | price | default
  -----------------------
  XYZ  |  20   | TRUE
  -----------------------
  XYZ  |  30   | FALSE
  -----------------------
  XYZ  |  40   | FALSE
  -----------------------
  ABC  | 50    | FALSE
  -----------------------
  </code></pre>
  
  <p>So in above table, for a particular <code>name</code> value, <code>default</code> value can be TRUE only once. And will have a unique constraint on name &amp; price columns.</p>
  
  <p>Is this possible?</p>