code: function (column, cb) {
              return this.__aggregateDataset().get(sql.max(column).minus(sql.min(column)), cb);
          }
docs: Returns a promise that is resolved with the interval between minimum and maximum values
  for the given column.
  
  @example
  // SELECT (max(id) - min(id)) FROM table LIMIT 1
  DB.from("table").interval("id").chain(function(interval){
  //(e.g) interval === 6
  });
  
  @param {String|patio.sql.Identifier} column to find the interval of.
  @param {Function} [cb] a function to be called when the aciton is complete
  
  @return {comb.Promise} a promise that will be resolved with the interval between the min and max values
  of the column.