Update if exists, otherwise insert (without unique keys) - mysql

Question

I have a table that is created like this:
'CREATE TABLE `boss_kills` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`characterid` int(11) DEFAULT NULL,
`mobid` int(11) DEFAULT NULL,
`amount` int(11) NOT NULL DEFAULT ''0'',
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=25 DEFAULT CHARSET=latin1'
Goal: I'm trying to create a system where a player kills a boss in the game and it records the boss's ID and the player's ID in the table. I want to be able to write one query where it updates if the player is already logged with the specific boss and inserts if he or she isn't.
Research: I did a lot of research online and people suggest the INSERT INTO ... ON DUPLICATE KEY UPDATE, but that only works if you either know your primary key or have a unique key, none of which I have or know.
Tries: I've tried
IF EXISTS (...) THEN UPDATE (...) ELSE INSERT(...)
and
UPDATE (...) IF ##ROWCOUNT = 0 INSERT INTO (...)
but they don't work. I get syntax errors.
If needed I can provide the errors thrown by the above tries. My current code for trying to update is this (but it throws a SQL syntax error):
Attempt 1:
UPDATE boss_kills
SET amount = amount + 1
WHERE characterid = ? AND mobid = ?
IF ##ROWCOUNT = 0
INSERT INTO boss_kills (characterid, mobid, amount) VALUES (?, ?, 1)
Attempt 2:
IF NOT EXISTS (SELECT id FROM boss_kills WHERE characterid = ? AND mobid = ?)
THEN
INSERT INTO boss_kills VALUES (DEFAULT, ?, ?, 1)
ELSE
UPDATE boss_kills SET amount = amount + 1 WHERE characterid = ? AND mobid = ?

Answer 1

It seems like (characterid, mobid) could make a unique index constraint for your table, thus allowing you to use INSERT ... ON DUPLICATE KEY UPDATE.
Use below script to create your table
CREATE TABLE boss_kills (
id int(11) NOT NULL AUTO_INCREMENT,
characterid int(11) DEFAULT NULL,
mobid int(11) DEFAULT NULL,
amount int(11) NOT NULL DEFAULT 0,
PRIMARY KEY ( id ),
UNIQUE ( characterid, mobid )
) ENGINE=MyISAM AUTO_INCREMENT=25 DEFAULT CHARSET=latin1;
Note that I've removed backticks around your table and column names - they are not necessary.
Inserting a row
INSERT INTO boss_kills (characterid, mobid, amount)
VALUES (?, ?, 1)
ON DUPLICATE KEY UPDATE amount = amount + 1;