title: MySql - apply same unique constraint to two different combinations of fields
body: <p>How do I specify unique constraint on <strong>different combinations</strong> of fields? E.g. </p>
  
  <pre><code> id  | Fld1 | Fld2 | Fld3
  -------------------------
    1  |   A  |  B   |  C
  -------------------------
    2  |   A  |  C   |  D
  </code></pre>
  
  <p>I'd like to make the example above illegal because combination (Fld1, Fld3) in row 1 has same values as combination (Fld1, Fld2) in row 2.  </p>
  
  <p>Is there any way to do this?</p>