title: sql: selecting rows, in which some values are equal (not all)
body: <p>Pre-condition:</p>
  
  <p>Table, like</p>
  
  <pre><code>id value1 value2 value3
  1   x       y      z
  2   x       y      x 
  3   x       z      y
  4   y       x      z
  5   x       x      z
  </code></pre>
  
  <p>Task: make query, which will select all rows, in which "pairs" (value1 and value2) are equal by rows (e.g. for our case result should be 1st and second row:</p>
  
  <pre><code>  id value1 value2 value3
      1   x       y      z
      2   x       y      x 
  </code></pre>
  
  <p>We do NOT know, what values could be in the table.</p>
  
  <p>So, this is the point.</p>