SELECT * FROM iso_code LEFT JOIN ( city LEFT JOIN country ON city.country_code  = country.code ) ON iso_code.alpha3 = country.code ;

alpha3	country_code	name	code	name

USA	USA	Austin	USA	United States

USA	USA	Boston	USA	United States

CAN	NULL	NULL	NULL	NULL

SELECT iso_code.alpha3 FROM iso_code LEFT JOIN ( city LEFT JOIN country ON city.country_code  = country.code ) ON iso_code.alpha3 = country.code ;

alpha3

USA

CAN

--source include/have_innodb.inc

CREATE TABLE iso_code (alpha3 VARCHAR(3)) ENGINE=InnoDB;

INSERT INTO iso_code VALUES ('USA'),('CAN');

CREATE TABLE city ( country_code VARCHAR(3), name VARCHAR(64)) ENGINE=InnoDB;

INSERT INTO city VALUES ('USA','Austin'),('USA','Boston');

CREATE TABLE country ( code VARCHAR(3),  name VARCHAR(64),  PRIMARY KEY (code),  UNIQUE KEY (name)) ENGINE=InnoDB;

INSERT INTO country VALUES ('CAN','Canada'),('USA','United States');

SELECT * FROM iso_code LEFT JOIN ( city LEFT JOIN country ON city.country_code  = country.code ) ON iso_code.alpha3 = country.code ;

SELECT iso_code.alpha3 FROM iso_code LEFT JOIN ( city LEFT JOIN country ON city.country_code  = country.code ) ON iso_code.alpha3 = country.code ;

DROP TABLE iso_code, city, country;

EXPLAIN EXTENDED

SELECT iso_code.alpha3 FROM iso_code LEFT JOIN ( city LEFT JOIN country ON city.country_code  = country.code ) ON iso_code.alpha3 = country.code ;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra

1	SIMPLE	iso_code	ALL	NULL	NULL	NULL	NULL	2	100.00	

Warnings:

Note	1003	select `test`.`iso_code`.`alpha3` AS `alpha3` from `test`.`iso_code` where 1