Optimize Join sentence with foreign keys, and show records with nulls

I have the following structure

SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";

CREATE TABLE IF NOT EXISTS `sis_param_tax` (
  `id` int(5) NOT NULL auto_increment,
  `description` varchar(50) NOT NULL,
  `code` varchar(5) NOT NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7;

CREATE TABLE IF NOT EXISTS `sis_param_city` (
  `id` int(4) NOT NULL auto_increment,
  `name` varchar(100) NOT NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;

CREATE TABLE IF NOT EXISTS `sis_supplier` (
  `id` int(15) NOT NULL auto_increment,
  `name` varchar(200) NOT NULL,
  `address` varchar(200) default NULL,
  `phone` varchar(30) NOT NULL,
  `fk_city` int(11) default NULL,
  `fk_tax` int(11) default NULL,
  PRIMARY KEY  (`id`),
  KEY `fk_city` (`fk_city`),
  KEY `fk_tax` (`fk_tax`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;

ALTER TABLE `sis_supplier`
  ADD CONSTRAINT `sis_supplier_ibfk_4` FOREIGN KEY (`fk_tax`) REFERENCES `sis_param_tax` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `sis_supplier_ibfk_3` FOREIGN KEY (`fk_city`) REFERENCES `sis_param_city` (`id`) ON DELETE SET NULL ON UPDATE CASCADE;

My questions are

1. This structure allows me to have a supplier with city and tax fields = null (in case user didn't set these values). Right?

2. If I delete "X" city, supplier's fk_city with city="X" are set to null, same with fk_tax. Right?

3. I want to optimize (IF POSSIBLE) the following join sentence, so I can show suppliers whom have fk_city and/or fk_tax = NULL

SELECT DISTINCT 
  sis_supplier.id,
  sis_supplier.name,
  sis_supplier.t开发者_运维百科elefono,
  sis_supplier.address,
  sis_supplier.phone,
  sis_supplier.cuit,
  sis_param_city.name AS city,
  sis_param_tax.description AS tax,
  sis_supplier.fk_city,
  sis_supplier.fk_tax
FROM
  sis_supplier 
  LEFT OUTER JOIN sis_param_city
  ON
  sis_supplier.`fk_city` = sis_param_city.id
  LEFT OUTER JOIN `sis_param_tax`
  ON
  sis_supplier.`fk_tax` = `sis_param_tax`.`id`

Thanks a lot in advance,

1. Yes.

2. Yes.

3. Yes, it's good to optimize. The query you showed looks fine. How is it not working for you?

Have you analyzed the query with EXPLAIN? This can help you tell when you have a query that isn't using indexes effectively. In fact, all of Chapter 7 Optimization would be recommended reading.

if you want to show records with nulls than use RIGHT or LEFT JOIN
depend on your needs