title: Postgres: convert single row to multiple rows (unpivot)
body: <p>I have a table:</p>
  
  <pre><code>Table_Name: price_list
  ---------------------------------------------------
  | id | price_type_a | price_type_b | price_type_c |
  ---------------------------------------------------
  | 1  |    1234      |     5678     |     9012     |
  | 2  |    3456      |     7890     |     1234     |
  | 3  |    5678      |     9012     |     3456     |
  ---------------------------------------------------
  </code></pre>
  
  <p>I need a select query in Postgres which gives result like this:</p>
  
  <pre><code>---------------------------
  | id | price_type | price |
  ---------------------------
  | 1  |  type_a    | 1234  |
  | 1  |  type_b    | 5678  |
  | 1  |  type_c    | 9012  |
  | 2  |  type_a    | 3456  |
  | 2  |  type_b    | 7890  |
  | 2  |  type_c    | 1234  |
  ...
  </code></pre>
  
  <p>Any help with links to similar examples greatly appreciated.</p>