Question

I have some MySQL code:

CREATE TABLE test_table (  
    id int(11) NOT NULL AUTO_INCREMENT,  
    count int(10) unsigned DEFAULT NULL,  
    PRIMARY KEY (id)  
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;

insert into test_table (count) values (1);
update test_table set count = IF( count -1 < 0, 0 , count -1 );
update test_table set count = IF( count -1 < 0, 0 , count -1 );

This should set count to zero if (count -1) < 0.

I get an error:

Error Code: 1264 Out of range value for column 'count' at row 1` instead.

This is due to count being an unsigned int. If I use a signed int it works.

Is this a bug in MySQL? I’m using: mysql Ver 14.14 Distrib 5.1.46sp1, for Win32 (ia32)

Answer 1

I think you should be doing this:

UPDATE test_table
SET count = IF( count < 1, 0 , count -1 )

The value of count - 1 underflows when count is zero.

SELECT count, count - 1 FROM test_table;
+-------+----------------------+
| count | count - 1            |
+-------+----------------------+
| 0     | 18446744073709551615 |
| 1     |                    0 |
| 2     |                    1 |
+-------+----------------------+

When I try your query, it doesn’t work… but it doesn’t fail in the same way as you see.