Source

File: wp-includes/class-wp-comment-query.php

protected function parse_orderby( $orderby ) {
		global $wpdb;

		$allowed_keys = array(
			'comment_agent',
			'comment_approved',
			'comment_author',
			'comment_author_email',
			'comment_author_IP',
			'comment_author_url',
			'comment_content',
			'comment_date',
			'comment_date_gmt',
			'comment_ID',
			'comment_karma',
			'comment_parent',
			'comment_post_ID',
			'comment_type',
			'user_id',
		);

		if ( ! empty( $this->query_vars['meta_key'] ) ) {
			$allowed_keys[] = $this->query_vars['meta_key'];
			$allowed_keys[] = 'meta_value';
			$allowed_keys[] = 'meta_value_num';
		}

		$meta_query_clauses = $this->meta_query->get_clauses();
		if ( $meta_query_clauses ) {
			$allowed_keys = array_merge( $allowed_keys, array_keys( $meta_query_clauses ) );
		}

		$parsed = false;
		if ( $this->query_vars['meta_key'] === $orderby || 'meta_value' === $orderby ) {
			$parsed = "$wpdb->commentmeta.meta_value";
		} elseif ( 'meta_value_num' === $orderby ) {
			$parsed = "$wpdb->commentmeta.meta_value+0";
		} elseif ( 'comment__in' === $orderby ) {
			$comment__in = implode( ',', array_map( 'absint', $this->query_vars['comment__in'] ) );
			$parsed      = "FIELD( {$wpdb->comments}.comment_ID, $comment__in )";
		} elseif ( in_array( $orderby, $allowed_keys, true ) ) {

			if ( isset( $meta_query_clauses[ $orderby ] ) ) {
				$meta_clause = $meta_query_clauses[ $orderby ];
				$parsed      = sprintf( 'CAST(%s.meta_value AS %s)', esc_sql( $meta_clause['alias'] ), esc_sql( $meta_clause['cast'] ) );
			} else {
				$parsed = "$wpdb->comments.$orderby";
			}
		}

		return $parsed;
	}