title: MySql, how to select not repeated values in 3 columns
body: <p>I have tried group by col1,col2,col3, but does not work because all values of the 3 columns cannot be repeated and cannot be equals other.</p>
  
  <p><strong>Table representation:</strong></p>
  
  <pre><code>+--------+--------+--------+
  | col1   | col2   | col3   |
  +--------+--------+--------+
  | value1 | value3 | value5 |
  +--------+--------+--------+
  | value1 | value2 | value1 |
  +--------+--------+--------+
  | value3 | value1 | value3 |
  +--------+--------+--------+
  </code></pre>
  
  <p>I need select values not repeated</p>
  
  <p><em>That table has 1+ million rows, performance is important when construct the query</em></p>
  
  <p>Thank you so much!</p>