@since 4.6.0
	 *
	 * @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'] );

		// Run after the 'get_terms_orderby' filter for backward compatibility.
		if ( $maybe_orderby_meta ) {
			$maybe_orderby_meta = $this->parse_orderby_meta( $_orderby );
			if ( $maybe_orderby_meta ) {
				$orderby = $maybe_orderby_meta;
			}
		}

		return $orderby;
	}

	/**
	 * Format response depending on field requested.
	 *
	 * @since 6.0.0
	 *
	 * @param WP_Term[] $term_objects Array of term objects.
	 * @param string    $_fields      Field to format.
	 *
	 * @return WP_Term[]|int[]|string[] Array of terms / strings / ints depending on field requested.
	 */
	protected function format_terms( $term_objects, $_fields ) {
		$_terms = array();
		if ( 'id=>parent' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[ $term->term_id ] = $term->parent;
			}
		} elseif ( 'ids' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[] = (int) $term->term_id;
			}
		} elseif ( 'tt_ids' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[] = (int) $term->term_taxonomy_id;
			}
		} elseif ( 'names' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[] = $term->name;
			}
		} elseif ( 'slugs' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[] = $term->slug;
			}
		} elseif ( 'id=>name' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[ $term->term_id ] = $term->name;
			}
		} elseif ( 'id=>slug' === $_fields ) {
			foreach ( $term_objects as $term ) {
				$_terms[ $term->term_id ] = $term->slug;
			}
		} elseif ( 'all' === $_fields || 'all_with_object_id' === $_fields ) {
			$_terms = $term_objects;
		}

		return $_terms;
	}

	/**
	 * Generate the ORDER BY clause for an 'orderby' param that is potentially related to a meta query.
	 *
	 * @since 4.6.0
	 *
	 * @param string $orderby_raw Raw 'orderby' value passed to WP_Term_Query.
	 * @return string ORDER BY clause.
	 */
	protected function parse_orderby_meta( $orderby_raw ) {
		$orderby = '';

		// Tell the meta query to generate its SQL, so we have access to table aliases.
		$this->meta_query->get_sql( 'term', 't', 'term_id' );
		$meta_clauses = $this->meta_query->get_clauses();
		if ( ! $meta_clauses || ! $orderby_raw ) {
			return $orderby;
		}

		$allowed_keys       = array();
		$primary_meta_key   = null;
		$primary_meta_query = reset( $meta_clauses );
		if ( ! empty( $primary_meta_query['key'] ) ) {
			$primary_meta_key = $primary_meta_query['key'];
			$allowed_keys[]   = $primary_meta_key;
		}
		$allowed_keys[] = 'meta_value';
		$allowed_keys[] = 'meta_value_num';
		$allowed_keys   = array_merge( $allowed_keys, array_keys( $meta_clauses ) );

		if ( ! in_array( $orderby_raw, $allowed_keys, true ) ) {
			return $orderby;
		}

		switch ( $orderby_raw ) {
			case $primary_meta_key:
			case 'meta_value':
				if ( ! empty( $primary_meta_query['type'] ) ) {
					$orderby = "CAST({$primary_meta_query['alias']}.meta_value AS {$primary_meta_query['cast']})";
				} else {
					$orderby = "{$primary_meta_query['alias']}.meta_value";
				}
				break;

			case 'meta_value_num':
				$orderby = "{$primary_meta_query['alias']}.meta_value+0";
				break;

			default:
				if ( array_key_exists( $orderby_raw, $meta_clauses ) ) {
					// $orderby corresponds to a meta_query clause.
					$meta_clause = $meta_clauses[ $orderby_raw ];
					$orderby     = "CAST({$meta_clause['alias']}.meta_value AS {$meta_clause['cast']})";
				}
				break;
		}

		return $orderby;
	}

	/**
	 * Parse an 'order' query variable and cast it to ASC or DESC as necessary.
	 *
	 * @since 4.6.0
	 *
	 * @param string $order The 'order' query variable.
	 * @return string The sanitized 'order' query variable.
	 */
	protected function parse_order( $order ) {
		if ( ! is_string( $order ) || empty( $order ) ) {
			return 'DESC';
		}

		if ( 'ASC' === strtoupper( $order ) ) {
			return 'ASC';
		} else {
			return 'DESC';
		}
	}

	/**
	 * Used internally to generate a SQL string related to the 'search' parameter.
	 *
	 * @since 4.6.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param string $search Search string.
	 * @return string Search SQL.
	 */
	protected function get_search_sql( $search ) {
		global $wpdb;

		$like = '%' . $wpdb->esc_like( $search ) . '%';

		return $wpdb->prepare( '((t.name LIKE %s) OR (t.slug LIKE %s))', $like, $like );
	}

	/**
	 * Creates an array of term objects from an array of term IDs.
	 *
	 * Also discards invalid term objects.
	 *
	 * @since 4.9.8
	 *
	 * @param Object[]|int[] $terms List of objects or term ids.
	 * @return WP_Term[] Array of `WP_Term` objects.
	 */
	protected function populate_terms( $terms ) {
		$term_objects = array();
		if ( ! is_array( $terms ) ) {
			return $term_objects;
		}

		foreach ( $terms as $key => $term_data ) {
			if ( is_object( $term_data ) && property_exists( $term_data, 'term_id' ) ) {
				$term = get_term( $term_data->term_id );
				if ( property_exists( $term_data, 'object_id' ) ) {
					$term->object_id = (int) $term_data->object_id;
				}
				if ( property_exists( $term_data, 'count' ) ) {
					$term->count = (int) $term_data->count;
				}
			} else {
				$term = get_term( $term_data );
			}

			if ( $term instanceof WP_Term ) {
				$term_objects[ $key ] = $term;
			}
		}

		return $term_objects;
	}

	/**
	 * Generate cache key.
	 *
	 * @since 6.2.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param array  $args WP_Term_Query arguments.
	 * @param string $sql  SQL statement.
	 *
	 * @return string Cache key.
	 */
	protected function generate_cache_key( array $args, $sql ) {
		global $wpdb;
		// $args can be anything. Only use the args defined in defaults to compute the key.
		$cache_args = wp_array_slice_assoc( $args, array_keys( $this->query_var_defaults ) );

		unset( $cache_args['cache_results'], $cache_args['update_term_meta_cache'] );

		if ( 'count' !== $args['fields'] && 'all_with_object_id' !== $args['fields'] ) {
			$cache_args['fields'] = 'all';
		}

		// Replace wpdb placeholder in the SQL statement used by the cache key.
		$sql = $wpdb->remove_placeholder_escape( $sql );

		$key          = md5( serialize( $cache_args ) . $sql );
		$last_changed = wp_cache_get_last_changed( 'terms' );
		return "get_terms:$key:$last_changed";
	}
}