public function group_by_expression( $function, $column ) {

		$column = $this->prepare_column( $column );

		$group = "{$function}($column)";

		if ( is_null( $this->group ) ) {
			$this->group = new Group( $group );
		} else {
			$this->group->then( $group );
		}

		return $this;
	}