title: Postgresql select distinct
body: <p>I'm with a problem on postgresql select.</p>
  
  <p>This query works fine:</p>
  
  <pre><code>SELECT
      DISTINCT(COLUMN_A || ' - ' || COLUMN_B),
      COLUMN_A,
      COLUMN_B
  FROM
      TABLE_A
  </code></pre>
  
  <p>But, when I include a new colmun on select like that:</p>
  
  <pre><code>SELECT
      DISTINCT(COLUMN_A || ' - ' || COLUMN_B),
      COLUMN_A,
      COLUMN_B,
      COLUMN_C
  FROM
      TABLE_A
  </code></pre>
  
  <p>The number of result grow's up, and the column with DISTINCT has repeated on result set.</p>
  
  <p>What is going on?</p>