Source

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

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

		$parsed = false;

		switch ( $orderby ) {
			case 'site__in':
				$site__in = implode( ',', array_map( 'absint', $this->query_vars['site__in'] ) );
				$parsed   = "FIELD( {$wpdb->blogs}.blog_id, $site__in )";
				break;
			case 'network__in':
				$network__in = implode( ',', array_map( 'absint', $this->query_vars['network__in'] ) );
				$parsed      = "FIELD( {$wpdb->blogs}.site_id, $network__in )";
				break;
			case 'domain':
			case 'last_updated':
			case 'path':
			case 'registered':
				$parsed = $orderby;
				break;
			case 'network_id':
				$parsed = 'site_id';
				break;
			case 'domain_length':
				$parsed = 'CHAR_LENGTH(domain)';
				break;
			case 'path_length':
				$parsed = 'CHAR_LENGTH(path)';
				break;
			case 'id':
				$parsed = "{$wpdb->blogs}.blog_id";
				break;
		}

		if ( ! empty( $parsed ) || empty( $this->meta_query_clauses ) ) {
			return $parsed;
		}

		$meta_clauses = $this->meta_query->get_clauses();
		if ( empty( $meta_clauses ) ) {
			return $parsed;
		}

		$primary_meta_query = reset( $meta_clauses );
		if ( ! empty( $primary_meta_query['key'] ) && $primary_meta_query['key'] === $orderby ) {
			$orderby = 'meta_value';
		}

		switch ( $orderby ) {
			case 'meta_value':
				if ( ! empty( $primary_meta_query['type'] ) ) {
					$parsed = "CAST({$primary_meta_query['alias']}.meta_value AS {$primary_meta_query['cast']})";
				} else {
					$parsed = "{$primary_meta_query['alias']}.meta_value";
				}
				break;
			case 'meta_value_num':
				$parsed = "{$primary_meta_query['alias']}.meta_value+0";
				break;
			default:
				if ( isset( $meta_clauses[ $orderby ] ) ) {
					$meta_clause = $meta_clauses[ $orderby ];
					$parsed      = "CAST({$meta_clause['alias']}.meta_value AS {$meta_clause['cast']})";
				}
		}

		return $parsed;
	}