MySQL ORDER BY strange sorting - mysql

Question

Problem: mysql query return results in strange order, looks like random or so. But it happens only with one hosting, localhost and another one hosting working well. Wanna get - why it happens and how to prevent it.
Schema:
CREATE TABLE `product` (
`product_id` int(11) NOT NULL AUTO_INCREMENT,
`sort_order` int(11) NOT NULL DEFAULT '0',
`status` tinyint(1) NOT NULL DEFAULT '0',
PRIMARY KEY (`product_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
insert into product (sort_order, status)
values
(0, 1),
(0, 1),
(0, 1),
(0, 1),
(0, 1);
CREATE TABLE `product_description` (
`product_id` int(11) NOT NULL,
`name` varchar(255) NOT NULL,
PRIMARY KEY (`product_id`),
KEY `name` (`name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
insert into product_description
values
(1, 'product_1'),
(2, 'product_2'),
(3, 'product_3'),
(4, 'product_4'),
(5, 'product_5');
CREATE TABLE `product_to_category` (
`product_id` int(11) NOT NULL,
`product_category_id` int(11) NOT NULL,
PRIMARY KEY (`product_id`,`product_category_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
insert into product_to_category
values
(1, 1),
(2, 1),
(3, 1),
(4, 1),
(5, 1);
CREATE TABLE `product_category_path` (
`product_category_id` int(11) NOT NULL,
`path_id` int(11) NOT NULL,
`level` int(11) NOT NULL,
PRIMARY KEY (`product_category_id`,`path_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
insert into product_category_path values (1, 1, 0);
Query:
SELECT p.product_id, pc.product_category_id, pd.name
FROM `product` p
LEFT JOIN `product_description` pd ON pd.product_id = p.product_id
LEFT JOIN `product_to_category` pc ON pc.product_id = p.product_id
WHERE p.status = 1 AND pc.product_category_id IN (SELECT product_category_id FROM `product_category_path` WHERE path_id = 1)
ORDER BY p.sort_order ASC;
On localhost and hosting result is always the same: 1,2,3,4,5. But on hosting it shows 1,3,2,5,4 or 2,1,5,3,4 and everytime new ordering. Why?
Update
http://dev.mysql.com/doc/refman/5.5/en/order-by-optimization.html
http://s.petrunia.net/blog/?p=24

Answer 1

SQL systems (of any make and model) are allowed to return result set rows in any order they find convenient unless you specify the order specifically. To put it another way, the order of a result set is formally unpredictable unless it's specified in ORDER BY. To put it a third way, on your localhost server, it's entirely accidental that your results are in the order you think they should be in. Tables have no inherent order.
You are really lucky your production server exposed this flaw in your query so quickly. Often developers don't find out about this stuff until their tables grow to tens of thousands of rows.
As this modification of your query shows (http://sqlfiddle.com/#!2/211536/2/0), all rows in your resultset have the same value of SORT_ORDER.
Query:
SELECT p.sort_order, p.product_id, pc.product_category_id, pd.name
FROM `product` p
LEFT JOIN `product_description` pd ON pd.product_id = p.product_id
LEFT JOIN `product_to_category` pc ON pc.product_id = p.product_id
WHERE p.status = 1 AND pc.product_category_id IN (SELECT product_category_id FROM `product_category_path` WHERE path_id = 1)
ORDER BY p.sort_order ASC
Results:
| SORT_ORDER | PRODUCT_ID | PRODUCT_CATEGORY_ID | NAME |
|------------|------------|---------------------|-----------|
| 0 | 1 | 1 | product_1 |
| 0 | 2 | 1 | product_2 |
| 0 | 3 | 1 | product_3 |
| 0 | 4 | 1 | product_4 |
| 0 | 5 | 1 | product_5 |
You've told SQL to order them that way. Both servers have done so.
If you need them to be ordered by PRODUCT_ID as well as SORT_ORDER just specify it (http://sqlfiddle.com/#!2/211536/4/0).
ORDER BY p.sort_order ASC, p.product_id ASC;