Update a field based off of the current value (in mysql) - mysql

Question

I have a table similar to
CREATE TABLE `mytable` (
`ID` int(11) unsigned NOT NULL AUTO_INCREMENT,
`Time` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
KEY `time` (`Time`),
) ENGINE=MyISAM AUTO_INCREMENT=2373485 DEFAULT CHARSET=latin1
I had a weird issue with daylight savings time, and now I need to update rows with IDs 2370144 through 2373391 so that the Time values are six hours less than their current values.
I can select the affected rows with
SELECT * FROM mytable WHERE ID >= 2370144 AND ID <= 2373391
How do I update these entries so that the new timestamp is six hours less than the old value?

Answer 1

I think this will work
UPDATE mytable SET Time = date_sub(Time, INTERVAL 6 HOUR) WHERE id BETWEEN 2370144 AND 2373391;

Answer 2

UPDATE mytable
SET `Time` = (`Time` - INTERVAL 6 HOUR)
WHERE ID >= 2370144
AND ID <= 2373391
Expanding on this a little bit, when feasible I would typically run a SQL query to generate a .sql file that contains one update statement per row, then execute that sql file to update the rows. Since you are only updating about 3,000 rows this should be feasible for you.
This dump and load approach has a couple of benefits:
You can save the SQL script as an
audit record of what you changed.
You can include both the ID and the Time
value in the SQL script. That way if
you accidentally run the script more
than once you don't end up changing
the value to something incorrect. For
example, if you ran my original
update twice the values would end up
6 hours too low, but if you use the
dump-and-load approach and run the
script twice, the second time it
won't change the records because the
where clause will no longer match.
Here's an example of the dump-and-load approach:
select concat('update mytable set `Time` = ''',
`Time` - interval 6 hour,
''' where id = ',
id,
' and `Time` = ''',
`Time`,
''';') as sql_stmt
into outfile '/tmp/mytable.update.dstfix.20110315.sql'
from mytable
WHERE ID >= 2370144
AND ID <= 2373391;
\. /tmp/mytable.update.dstfix.20110315.sql