Using WHERE clause in ON DUPLICATE KEY UPDATE column - mysql

Question

Here's the table design for a cartitems table
cartitems
cartitem_id PK AI
customer_id FK
item_id FK UNIQUE
item_quantity INT DEFAULT 1
What I need to accomplish
1.) if item_id exists in the table, increment the item_quantity by 1 everytime user clicks on "Add To Cart" button for the same item_id
2.) if item_id doesn't yet exist in the table, run an insert statement.
So what I did is this.
CREATE DEFINER=`root`#`localhost` PROCEDURE `addItemToCart`(aCustomerId int, aProductId int)
BEGIN
INSERT INTO cart_items(customer_id,product_id)
VALUES(aCustomerId,aProductId)
ON DUPLICATE KEY UPDATE
item_quantity = item_quantity + 1
WHERE customer_id = aCustomerId AND item_id = aProductId;
END
But when I inspected it, I get an error that says, missing semicolon at item_quantity = item_quantity + 1
I can't figure out what's causing the error. I don't know if it's a problem with the WHERE clause.
I'd appreciate any help.
Thanks.

Answer 1

The behavior of ON DUPLICATE KEY UPDATE is well explained in the documentation:
If you specify ON DUPLICATE KEY UPDATE, and a row is inserted that would cause a duplicate value in a UNIQUE index or PRIMARY KEY, MySQL performs an UPDATE of the old row
Looking at your table, you want to increment the quantity of an item for a given customer's cart. I am assuming here that a customer may only have one shopping cart at a time. So the conditions under which you want MySQL to perform an UPDATE rather than an INSERT is when the customer and item already appear in the table.
Towards this end, you can create a unique index on these two columns via:
CREATE UNIQUE INDEX unq_item_index ON cart_items (customer_id, item_id);
Then, you can use the following INSERT query:
INSERT INTO cart_items (customer_id, item_id)
VALUES
(aCustomerId, anItemId)
ON DUPLICATE KEY UPDATE
item_quantity = item_quantity + 1
WHERE customer_id = aCustomerId AND
item_id = anItemId;
Now the behavior will be that if a new entry for customer/item comes in, the item_quantity will be set to the default value of 1, otherwise the item_quantity will be incremented by 1.

Answer 2

BEGIN
IF NOT EXISTS(SELECT 1 FROM cart_items WHERE item_id = aProductId)
BEGIN
INSERT INTO cart_items(customer_id,product_id)
VALUES(aCustomerId,aProductId)
END
ELSE
UPDATE cart_items SET item_quantity = item_quantity + 1 WHERE customer_id = aCustomerId AND item_id = aProductId;
END

Answer 3

Why are you trying to use that WHERE clause there? It doesn't make sense. If DUPLICATE KEY happens it will UPDATE whatever field you specified to the old record that has the same key. So, if you just remove WHERE clause it should work. Check this article. It states the following two:
INSERT INTO table (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE c=c+1;
UPDATE table SET c=c+1 WHERE a=1;
are the same.