WP_Term_Query::parse_orderby

	/**
	 * Parse and sanitize 'orderby' keys passed to the term query.
	 *
	 * @since 4.6.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param string $orderby_raw Alias for the field to order by.
	 * @return string|false Value to used in the ORDER clause. False otherwise.
	 */
	protected function parse_orderby( $orderby_raw ) {
		$_orderby           = strtolower( $orderby_raw );
		$maybe_orderby_meta = false;

		if ( in_array( $_orderby, array( 'term_id', 'name', 'slug', 'term_group' ), true ) ) {
			$orderby = "t.$_orderby";
		} elseif ( in_array( $_orderby, array( 'count', 'parent', 'taxonomy', 'term_taxonomy_id', 'description' ), true ) ) {
			$orderby = "tt.$_orderby";
		} elseif ( 'term_order' === $_orderby ) {
			$orderby = 'tr.term_order';
		} elseif ( 'include' === $_orderby && ! empty( $this->query_vars['include'] ) ) {
			$include = implode( ',', wp_parse_id_list( $this->query_vars['include'] ) );
			$orderby = "FIELD( t.term_id, $include )";
		} elseif ( 'slug__in' === $_orderby && ! empty( $this->query_vars['slug'] ) && is_array( $this->query_vars['slug'] ) ) {
			$slugs   = implode( "', '", array_map( 'sanitize_title_for_query', $this->query_vars['slug'] ) );
			$orderby = "FIELD( t.slug, '" . $slugs . "')";
		} elseif ( 'none' === $_orderby ) {
			$orderby = '';
		} elseif ( empty( $_orderby ) || 'id' === $_orderby || 'term_id' === $_orderby ) {
			$orderby = 't.term_id';
		} else {
			$orderby = 't.name';

			// This may be a value of orderby related to meta.
			$maybe_orderby_meta = true;
		}

		/**
		 * Filters the ORDERBY clause of the terms query.
		 *
		 * @since 2.8.0
		 *
		 * @param string   $orderby    `ORDERBY` clause of the terms query.
		 * @param array    $args       An array of term query arguments.
		 * @param string[] $taxonomies An array of taxonomy names.
		 */
		$orderby = apply_filters( 'get_terms_orderby', $orderby, $this->query_vars, $this->query_vars['taxonomy'] );