code: function(db, callback) {
  
    // Check Cache for Link Categories
    let cacheKey = db.id + "-" + 'categories';
    let cache = cache_linkCategories.get(cacheKey);
    if ( cache !== null ) {
      return callback(null, cache);
    }
  
    // Build the select statement
    let select = "SELECT DISTINCT link_category_title FROM rt_links";
  
    // Query the database
    db.select(select, function(err, results) {
  
      // Database Query Error
      if ( err ) {
        return callback(err);
      }
  
      // List of categories to return
      let rtn = [];
  
      // Parse each row
      for ( let i = 0; i < results.length; i++ ) {
        let row = results[i];
        rtn.push(row.link_category_title);
      }
  
      // Add categories to cache
      cache_linkCategories.put(cacheKey, rtn);
  
      // Return the categories with the callback
      return callback(null, rtn);
  
    });
  
  }
docs: ==== QUERY FUNCTIONS ==== // 
  Get a list of all link category titles
  
  @param {RightTrackDB} db The Right Track DB to query
  @param {function} callback Callback function
  @param {Error} callback.error Database Query Error
  @param {string[]} [callback.categories] The selected Link categories