I need to solve a problem similar to the language one whereby if a
translation in the chosen languange does not exist, the default one should
be selected. My problem is actually picking a price specific to a certain
class if available or a default price otherwise.

So taking a table like:

CREATE TABLE `price_table` (
`class` varchar(1) NOT NULL,
`item` varchar(1) NOT NULL,
`price` int(11) NOT NULL,
PRIMARY KEY (`item`,`class`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

INSERT INTO `price_table` (`class`, `item`, `price`) VALUES
('', 'A', 50),
('1', 'A', 60),
('', 'B', 30),
('', 'C', 20),
('2', 'C', 0);

to find the price of item A I could do:

SELECT
`price`
FROM `price_table`
WHERE `item` = '$item' AND (`class` = '$class' OR `class` = '')
ORDER BY `class` DESC
LIMIT 1

so if $item was 'A' $class was '1', I would get 60, otherwise I will get 50.

Or I could do

SELECT
COALESCE(`s`.`price`,`d`.`price`) `price`
FROM `price_table` `d`
LEFT JOIN `price_table` `s` ON `s`.`item` = `d`.`item` AND `s`.`class` =
'$class'
WHERE `d`.`item` = '$item' AND `d`.`class` = ''

My inclination is to go with the LEFT JOIN, but what do others think?