Source

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

protected function parse_search_order( &$q ) {
		global $wpdb;

		if ( $q['search_terms_count'] > 1 ) {
			$num_terms = count( $q['search_orderby_title'] );

			// If the search terms contain negative queries, don't bother ordering by sentence matches.
			$like = '';
			if ( ! preg_match( '/(?:\s|^)\-/', $q['s'] ) ) {
				$like = '%' . $wpdb->esc_like( $q['s'] ) . '%';
			}

			$search_orderby = '';

			// Sentence match in 'post_title'.
			if ( $like ) {
				$search_orderby .= $wpdb->prepare( "WHEN {$wpdb->posts}.post_title LIKE %s THEN 1 ", $like );
			}

			// Sanity limit, sort as sentence when more than 6 terms
			// (few searches are longer than 6 terms and most titles are not).
			if ( $num_terms < 7 ) {
				// All words in title.
				$search_orderby .= 'WHEN ' . implode( ' AND ', $q['search_orderby_title'] ) . ' THEN 2 ';
				// Any word in title, not needed when $num_terms == 1.
				if ( $num_terms > 1 ) {
					$search_orderby .= 'WHEN ' . implode( ' OR ', $q['search_orderby_title'] ) . ' THEN 3 ';
				}
			}

			// Sentence match in 'post_content' and 'post_excerpt'.
			if ( $like ) {
				$search_orderby .= $wpdb->prepare( "WHEN {$wpdb->posts}.post_excerpt LIKE %s THEN 4 ", $like );
				$search_orderby .= $wpdb->prepare( "WHEN {$wpdb->posts}.post_content LIKE %s THEN 5 ", $like );
			}

			if ( $search_orderby ) {
				$search_orderby = '(CASE ' . $search_orderby . 'ELSE 6 END)';
			}
		} else {
			// Single word or sentence search.
			$search_orderby = reset( $q['search_orderby_title'] ) . ' DESC';
		}

		return $search_orderby;
	}