title: Creating database table when installing a wordpress plugin
body: <p>I want to create a table into the database upon installing the plugin I've created.</p>
  
  <p>In my main plugin file (index.php): </p>
  
  <pre><code>register_activation_hook(__FILE__, 'wnm_install');
  
  global $wnm_db_version;
  $wnm_db_version = "1.0";
  
  function wnm_install(){
  global $wpdb;
  global $wnm_db_version;
  $sql = "CREATE TABLE tbl_campaigns (
  campaignID int(11) NOT NULL AUTO_INCREMENT,
  campaign_name varchar(128) NOT NULL,
  start_duration date NOT NULL,
  end_duration date NOT NULL,
  activity varchar(500) NOT NULL,
  survey_settings varchar(50) NOT NULL,
  limit varchar(50) NOT NULL,
  goal varchar(100) DEFAULT NULL,
  PRIMARY KEY (campaignID)
  ) ;";
  
  require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
  dbDelta($sql);
  add_option("wnm_db_version", $wnm_db_version);
  }
  </code></pre>
  
  <p>I just followed the instructions from this <a href="http://codex.wordpress.org/Creating_Tables_with_Plugins" rel="nofollow noreferrer">http://codex.wordpress.org/Creating_Tables_with_Plugins</a></p>
  
  <p>But it doesn't work.</p>
  
  <p>What seems to be the problem with this code?</p>