Question

I came across this strange behavior when I was hunting for a bug in a system. Consider following.

We have a mysql table which have varchar(100) column. See the following sql script.

create table user(`id` bigint(20) NOT NULL AUTO_INCREMENT,`user_id` varchar(100) NOT NULL,`username` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`),UNIQUE KEY `user_id` (`user_id`))  ENGINE=InnoDB AUTO_INCREMENT=129 DEFAULT CHARSET=latin1;

insert into user(user_id, username) values('20120723145614834', 'user1');

insert into user(user_id, username) values('20120723151128642', 'user1');

When I execute following query I received 0 results.

select * from user where user_id=20120723145614834;

But When I execute following I get the result(note the single quote).

select * from user where user_id='20120723145614834';

This is expected since user_id field is varchar. Strange thing is that both following queries yield result.

select * from user where user_id=20120723151128642;
select * from user where user_id='20120723151128642';

Can anybody explain me the reason for this strange behavior. My MySql version is 5.1.63-0ubuntu0.11.10.1

Answer 1

Check mysql document 12.2. Type Conversion in Expression Evaluation

Comparisons that use floating-point numbers (or values that are
converted to floating-point numbers) are approximate because such
numbers are inexact. This might lead to results that appear
inconsistent:

mysql> SELECT '18015376320243458' = 18015376320243458;
        -> 1
mysql> SELECT '18015376320243459' = 18015376320243459;
        -> 0

So we better use always right data type for SQL.