First, here’s the concise summary of the question:
Is it possible to run an
INSERT statement conditionally?
Something akin to this:
Now, I know I can do this with a stored procedure.
My question is: can I do this in my query?
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
So far so good.
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
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.
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
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!
INSERT INTO orders(product_id, qty) SELECT 2, 20 FROM products WHERE id = 2 AND qty_on_hand >= 20
If a product with
id equal to
2 exists and the
qty_on_hand is greater or equal to
20 for this product, then an insert will occur with the values
product_id = 2, and
qty = 20. Otherwise, no insert will occur.
Note: If your product ids are note unique, you might want to add a
LIMIT clause at the end of the
You’re probably solving the problem the wrong way.
If you’re afraid two read-operations will occur at the same time and thus one will work with stale data, the solution is to use locks or transactions.
Have the query do this:
- lock table for read
- read table
- update table
- release lock
Not sure about concurrency, you’ll need to read up on locking in mysql, but this will let you be sure that you only take 20 items if 20 items are available:
update products set qty_on_hand = qty_on_hand - 20 where qty_on_hand >= 20 and id=2
You can then check how many rows were affected. If none were affected, you did not have enough stock. If 1 row was affected, you have effectively consumed the stock.