SELECT pr.sid, zz.minprice, zz.pid FROM (
SELECT c.pid, MIN(p.price) AS minprice, p.sid
FROM coupling AS c
LEFT JOIN price AS p ON p.id = c.spid
GROUP BY c.pid) zz
JOIN coupling AS cp ON zz.pid = cp.pid
JOIN price AS pr ON pr.price = zz.minprice AND pr.id = cp.spid

CREATE TABLE `coupling` (
	`id` INT(11) NOT NULL AUTO_INCREMENT,
	`pid` INT(11) NOT NULL,
	`spid` INT(11) NOT NULL,
	PRIMARY KEY (`id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=6
;

CREATE TABLE `price` (
	`id` INT(11) NOT NULL AUTO_INCREMENT,
	`price` INT(11) NOT NULL,
	`name` VARCHAR(50) NOT NULL,
	`sid` INT(11) NOT NULL,
	PRIMARY KEY (`id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=541
;