title: SQL selecting column depending on the value of another column
body: <p>Consider the following table.</p>
  
  <pre>
  -----------------------------------------------------
  | game_id | blue_player_id | red_player_id | turn_of |
  -----------------------------------------------------
  |    1    |        4       |        5      |  "blue" |
  -----------------------------------------------------
  |    2    |        1       |        2      |  "red"  |
  -----------------------------------------------------
  |    3    |        3       |        4      |  "blue" |
  -----------------------------------------------------
  </pre>
  
  <p>I want to write a query that selects either the blue_player_id or the red_player_id depending on the turn_of column for a given game id. </p>
  
  <p>Thus for game_id=1 it should return 4, for game_id=2 it should return 2, and for game_id=3 it should return 3.</p>
  
  <p>I know I could do this in two query by first selecting who's turn it is and then selecting the right column in the second query. However, is there a one query method which does not use database specific SQL?</p>