Question

I was confused, when I figured out that setting value to 0 actually picks some row. O_o
0 is number, not a string…
It’s something i didn’t knew? Some specific feature of mysql or i’m doing something wrong?

Here is query:

SELECT * FROM contacts WHERE owner=0

The result is:

contact_id,owner,contact
10,d9659deb29f21dcf468783f1e7f52aa2b2ab6b48,7d1d00cd5cc06c27b3bcbefa8b4aeeb16bdec14e

Here is test dump:

-- --------------------------------------------------------
-- Host:                         127.0.0.1
-- Server version:               5.5.8 - Source distribution
-- Server OS:                    Linux
-- HeidiSQL version:             7.0.0.4053
-- Date/time:                    2012-08-02 11:05:48
-- --------------------------------------------------------

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET NAMES utf8 */;
/*!40014 SET FOREIGN_KEY_CHECKS=0 */;

-- Dumping database structure for test
CREATE DATABASE IF NOT EXISTS `test` /*!40100 DEFAULT CHARACTER SET utf8 */;
USE `test`;


-- Dumping structure for table test.contacts
CREATE TABLE IF NOT EXISTS `contacts` (
  `contact_id` int(10) NOT NULL AUTO_INCREMENT,
  `owner` varchar(40) DEFAULT '',
  `contact` varchar(40) DEFAULT '',
  PRIMARY KEY (`contact_id`),
  KEY `owner` (`owner`),
  KEY `contact` (`contact`)
) ENGINE=InnoDB AUTO_INCREMENT=11 DEFAULT CHARSET=utf8;

-- Dumping data for table test.contacts: ~2 rows (approximately)
/*!40000 ALTER TABLE `contacts` DISABLE KEYS */;
REPLACE INTO `contacts` (`contact_id`, `owner`, `contact`) VALUES
    (9, '7d1d00cd5cc06c27b3bcbefa8b4aeeb16bdec14e', 'd9659deb29f21dcf468783f1e7f52aa2b2ab6b48'),
    (10, 'd9659deb29f21dcf468783f1e7f52aa2b2ab6b48', '7d1d00cd5cc06c27b3bcbefa8b4aeeb16bdec14e');
/*!40000 ALTER TABLE `contacts` ENABLE KEYS */;
/*!40014 SET FOREIGN_KEY_CHECKS=1 */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;

Answer 1

The owner ‘7d1d00cd5cc06c27b3bcbefa8b4aeeb16bdec14e’ is started with a number ‘7…’, MySQL converts this value to 7, we have 7 = 0 – FALSE.

The owner ‘d9659deb29f21dcf468783f1e7f52aa2b2ab6b48’ is started with a char ‘d…’, MySQL converts this value to 0, so 0 = 0 – TRUE.

This is how MySQL converts strings to numbers.