concurrency - MySQL INSERT IF (custom if statements)

Question

First, here's the concise summary of the question:

Is it possible to run an INSERT statement conditionally? Something akin to this:

IF(expression) INSERT...

Now, I know I can do this with a stored procedure. My question is: can I do this in my query?

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Now, why would I want to do that?

Let's assume we have the following 2 tables:

products: id, qty_on_hand
orders: id, product_id, qty

Now, let's say an order for 20 Voodoo Dolls (product id 2) comes in.
We first check if there's enough Quantity On Hand:

SELECT IF(
    ( SELECT SUM(qty) FROM orders WHERE product_id = 2  ) + 20
    <=
    ( SELECT qty_on_hand FROM products WHERE id = 2)
, 'true', 'false');

Then, if it evaluates to true, we run an INSERT query.
So far so good.

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However, there's a problem with concurrency.
If 2 orders come in at the exact same time, they might both read the quantity-on-hand before any one of them has entered the order. They'll then both place the order, thus exceeding the qty_on_hand.

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So, back to the root of the question:
Is it possible to run an INSERT statement conditionally, so that we can combine both these queries into one?

I searched around a lot, and the only type of conditional INSERT statement that I could find was ON DUPLICATE KEY, which obviously does not apply here.

Answer 1

INSERT INTO TABLE
SELECT value_for_column1, value_for_column2, ...
FROM wherever
WHERE your_special_condition

If no rows are returned from the select (because your special condition is false) no insert happens.

Using your schema from question (assuming your id column is auto_increment):

insert into orders (product_id, qty)
select 2, 20
where (SELECT qty_on_hand FROM products WHERE id = 2) > 20;

This will insert no rows if there's not enough stock on hand, otherwise it will create the order row.

Nice idea btw!