How to create an all-or-nothing update query in Postgres?

Question

I have the following query:

UPDATE items SET quantity = quantity - 1 
WHERE quantity > 0 AND user_id = $1 AND item_id IN (5, 6, 7);

I'd like to modify it such that the update will only occur if all three rows are updated.

That is, unless that user has items 5, 6, 7 with quantities greater than 0 for each of them, 0 rows will be updated. However, if the condition is true for each, then all three rows are updated.

I'm not sure of a simple way to do this. My gut solution is to use a CTE where the initial query gets the COUNT and then you only perform the update if the count = 3, but I think there must be a better way?

Also, I am using 3 items here as an example. The number of item_ids is variable, and can be anywhere between 1 and 20 in my case (passed from the app server as an array)

Answer 1

Use transaction. Inside the transaction, execute the UPDATE . Check the number of rows updated. If that number is less than the length of the list of IDs, abort the transaction with ROLLBACK , else COMMIT .

Answer 2

Yet another option is to check when the couple of <user_id, item_id> is not present with a quantity equal to 0, using the NOT EXISTS operator.

UPDATE items i
SET quantity = quantity - 1 
WHERE user_id = $1 
  AND item_id IN (5, 6, 7)
  AND NOT EXISTS (SELECT user_id, item_id 
                  FROM items 
                  WHERE i.user_id = user_id AND i.item_id = item_id 
                    AND quantity = 0);

Check the demo here .

Answer 3

Added a check constraint to the table quantity >= 0 and then just did this:

UPDATE items SET quantity = quantity - 1 
WHERE user_id = $1 AND item_id IN (5, 6, 7);