Insert or update based on row data - mysql

Question

First off, I know about REPLACE INTO and INSERT INTO ... ON KEY DUPLICATE UPDATE but this is not what I'm looking for -or- I don't know how to use them to achieve what I want.
This is my simple table structure:
+-----------+---------+----------+----------+
| player_id | item_id | quantity | location |
+-----------+---------+----------+----------+
My INSERT query looks like this:
INSERT INTO items VALUES (2, 10, 40, 1);
Now, if there is a row where all fields match, except for quantity (doesn't matter if it matches or not, but the point is that the other 3 match). So, if there's a row where player_id is 2, item_id is 10 and location is 1 (quantity value doesn't matter - it can be 40, but also doesn't have to), then I want to update it, rather than insert a new one.
Obviously, I'm looking for a way that is different than SELECT + UPDATE, if there is any...

Answer 1

If there are no other constraints to be considered, couldn't you just add a combined unique key over (player_id, item_id and location), and then go for INSERT INTO ... ON DUPLICATE KEY UPDATE?
Edit: Trying to clarify. I suppose you have something like the following table creation statement:
CREATE TABLE items (
player_id INT NOT NULL,
item_id INT NOT NULL,
quantity INT NOT NULL,
location INT NOT NULL
) ENGINE = InnoDB CHARACTER SET utf8 COLLATE utf8_unicode_ci;
You add a combined unique index for three columns:
ALTER TABLE items ADD UNIQUE player_item_location (player_id, item_id, location);
So you can INSERT this row:
INSERT INTO items (player_id, item_id, quantity, location) VALUES (2, 10, 40, 1);
And if you try to execute the same INSERT again, you end up with the message:
#1062 - Duplicate entry '2-10-1' for key 'player_item_location'
But if you add the ON DUPLICATE KEY UPDATE like this:
INSERT INTO items (player_id, item_id, quantity, location) VALUES (2, 10, 30, 1) ON DUPLICATE KEY UPDATE quantity = 30;
You will end up in not adding another row, but updating the existing one (player 2, item 10, location 1) and changing its quantity from 40 to 30.
And, if you want to add another row, say for player 3, item 10, location 1, this will work, too:
INSERT INTO items (player_id, item_id, quantity, location) VALUES (3, 10, 40, 1);
So after the three INSERTs, you should end up in having the following rows in your table:
mysql> SELECT * FROM items;
+-----------+---------+----------+----------+
| player_id | item_id | quantity | location |
+-----------+---------+----------+----------+
| 2 | 10 | 30 | 1 |
| 3 | 10 | 40 | 1 |
+-----------+---------+----------+----------+
2 rows in set (0.00 sec)
Based on your question, I thought that this is the behaviour you wanted to have. If not, please let us know what exactly doesn't work or where I didn't understand you correctly.

Answer 2

Well, you can use BEFORE INSERT triggers, which will be executed before every insert and make the required changes according to the values.
Read more here: http://dev.mysql.com/doc/refman/5.0/en///create-trigger.html