Locking database table between query and insert - sql

Question

Forgive me if this is a silly question (I'm new to databases and SQL), but is it possible to lock a table, similar to the lock keyword in C#, so that I can query the database to see if a condition is met, then insert a row afterwards while ensuring the state of the table has not changed between the two actions?
In this case, I have a table transactions which has two columns: user and product. This is a many-to-one relationship; multiple users can have the same product. However, the number of products is limited.
When a user adds a product to their account, I want to first check if the total number of items with the same product value to see if it is under a certain threshold, then add the transaction afterwards. However, since this is a multithreaded application, multiple transactions can come in at the same time. I want to make sure that one of these is rejected, and one succeeds, such that the number of transactions with the same product value can never be higher than the limit.
Rough pseudo-code for what I am trying to do:
my_user, my_product = ....
my_product_count = 0
for each transaction in transactions:
if product == my_product:
my_product_count += 1
if my_product_count < LIMIT:
insert my_user, my_product into transactions
return SUCCESS
else:
return FAILURE
I am using SQLAlchemy with SQLite3, if that matters.

Answer 1

Not needed if you do both operations in a transaction - which is supported by databases. Databases do maintain locks to guarantee transactional integrity. In fact that is one of the four pillars of what a database does - they are called ACID guaranetees (for (Atomicity, Consistency, Isolation, Durability).
So, in your case, to ensure consistence you would make both operations in one transaction and seat the transaction parameters in such a way to block reads on the already read rows.
SQL locking is WAY more powerfull than the lock statement because, among other things, databases per definition have multiple threads (users) hitting the same data - something that is exceedingly rare in programming (where same data access is avoided in multi threaded programming as much as possible).
I suggest a good book about SQL - because you need to simply LEARN some fundamental concepts at one point, or you will make mistakes that cost money.

Answer 2

Transactions allow you to use multiple SQL statements atomically.
(SQLite implements transactions by locking the entire database, but the exact mechanism doesn't matter, and you might want to use another database later anyway.)
However, you don't even need to bother with explicit transactions if your desired algorithm can be handled with single SQL statement, like this:
INSERT INTO transactions(user, product)
SELECT #my_user, #my_product
WHERE (SELECT COUNT(*)
FROM transactions
WHERE product = #my_product) < #LIMIT;