code: public function addOrderBy($columns)
      {
          $columns = $this->normalizeOrderBy($columns);
          if ($this->orderBy === null) {
              $this->orderBy = $columns;
          } else {
              $this->orderBy = array_merge($this->orderBy, $columns);
          }
  
          return $this;
      }
docs: Adds additional ORDER BY columns to the query.
  @param string|array|ExpressionInterface $columns the columns (and the directions) to be ordered by.
  Columns can be specified in either a string (e.g. "id ASC, name DESC") or an array
  (e.g. `['id' => SORT_ASC, 'name' => SORT_DESC]`).
  
  The method will automatically quote the column names unless a column contains some parenthesis
  (which means the column contains a DB expression).
  
  Note that if your order-by is an expression containing commas, you should always use an array
  to represent the order-by information. Otherwise, the method will not be able to correctly determine
  the order-by columns.
  
  Since version 2.0.7, an [[ExpressionInterface]] object can be passed to specify the ORDER BY part explicitly in plain SQL.
  @return $this the query object itself
  @see orderBy()