MySQL INSERT and UPDATE with IF condition - mysql

Question

I have a simple MySQL Database like:
CREATE TABLE `tab_update` (
`id` int(11) NOT NULL,
`number` int(11) NOT NULL,
`description` varchar(11) NOT NULL,
UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
and some data inside:
INSERT INTO `tab_update` (`id`, `number`, `description`) VALUES ('111', '5', 'first value');
INSERT INTO `tab_update` (`id`, `number`, `description`) VALUES ('222', '5', 'first value');
INSERT INTO `tab_update` (`id`, `number`, `description`) VALUES ('333', '5', 'first value');
What I want is to:
insert data to the database, if the same 'id' does not exist already
or
update data if both below conditions are met:
a) 'id' is already in the database
b) new number value is bigger than the number stored already
What I am trying to do now is:
INSERT INTO tab_update (id, number, description) VALUES (333, 1, 'second value')
ON DUPLICATE KEY UPDATE number = GREATEST(number, VALUES(number)), description = 'second value'
this works partially - inserts data or update, however, I have a wrong behavior. If we have 'number = 1' (which is less then 5) this should not update the record.
At the moment keeps updating description, but should not.
Could you please guys help me improve this query?
update:
I am trying also:
INSERT INTO tab_update (id, number, description) VALUES (333, 6, 'second value')
ON DUPLICATE KEY UPDATE
number = GREATEST(number, VALUES(number)),
description = IF(VALUES(number) > number, VALUES(description), description)
but last line does not work as expected.