MySQL: Complete missing data with parent tables - php

Question

Let's assume I have three tables in my database, representing a hierarchic order:
countries, states, and cities
Each city is connected to one state, each state to one country. This is simple to represent in a database.
Let's further assume each of those tables contains a field tax_rate. In a basic case the tax rate is defined on country level and null on all other levels. However, it could be overwritten on any of the levels below.
When I query for a city node I would like to get its tax rate. It could be defined right within the same node, but more likely it will be defined on any of the next-higher levels.
What is the most efficient way to accomplish this either in MySQL or on PHP level? In my real life application there will not be only one such field but many of them.
Below is a simple database schema of my example. Of course it would also have foreign key definitions.
CREATE TABLE `countries` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`tax_rate` float(4,2) DEFAULT NULL,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO `countries` (`id`, `tax_rate`, `name`)
VALUES
(1,8.00,'Switzerland'),
(2,16.00,'Germany');
CREATE TABLE `cities` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`state_id` int(11) DEFAULT NULL,
`tax_rate` float(4,2) DEFAULT NULL,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
)
NSERT INTO `cities` (`id`, `state_id`, `tax_rate`, `name`)
VALUES
(1,1,NULL,'Bern'),
(2,1,NULL,'Zollikofen'),
(3,2,NULL,'Zurich'),
(4,2,5.30,'Wettingen'),
(5,2,NULL,'Winterthur'),
(6,2,6.60,'Handligen'),
(7,3,NULL,'Bayern-Town 1'),
(8,3,NULL,'Bayern-Town 2'),
(9,3,9.00,'Bayern-Town 3');
CREATE TABLE `states` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`country_id` int(11) DEFAULT NULL,
`tax_rate` float(4,2) DEFAULT NULL,
`name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO `states` (`id`, `country_id`, `tax_rate`, `name`)
VALUES
(1,1,NULL,'Bern'),
(2,1,9.00,'Zurich'),
(3,2,NULL,'Bavaria');

Answer 1

Use COALESCE(). That's what it's for.

Answer 2

This could be handled on either level: MySQL or PHP
I'd prefer the MySQL approach:
select cities.name, COALESCE(cities.tax_rate,states.tax_rate,countries.tax_rate) from cities
join states on cities.state_id=states.id
join countries on states.country_id = countries.id
This will return the city's tax rate if it is not NULL, else the state's. If that also is null, it'll return the country's tax rate.