Rollback transaction not working - sql-server

Question

2 tables with data image
I have two tables tblproduct and tblproductsales.
I want to see how Begin Tran, Rollback work.
There is one insert statement and update statement in the stored proc. If either one of statements fail, both should rollback. but rollback is not working even if Update statement is failing. any suggestion please?
CREATE PROCEDURE spbasam_ProductSales
#ProductId int,
#QtyNeeded int
AS
BEGIN
--Check the if you have enough stock to see
DECLARE #productavailabiltycount int
SELECT #productavailabiltycount = QtyAvailable FROM dbo.tblProduct where ProductId = #ProductId
--check to see if you have enough
If (#productavailabiltycount < #QtyNeeded)
BEGIN
Raiserror('Not enough stock available',16,1)
END
Else
BEGIN
BEGIN TRY
BEGIN TRAN
--Step 1 to reduce tblProduct table
DECLARE #toupdate int
UPDATE dbo.tblProduct set QtyAvailable = #productavailabiltycount - #QtyNeeded
WHERE ProductId = #ProductId
-- Step 2 insert into tblProductSales table
--First get the max count of the productSalesId
DECLARE #maxcountid int
SELECT #maxcountid = MAX(ProductSalesId) from dbo.tblProductSales
INSERT INTO dbo.tblProductSales Values(#maxcountid+1 , #ProductId, #QtyNeeded)
COMMIT TRAN
END TRY
BEGIN CATCH
Rollback Transaction
SELECT
ERROR_NUMBER() as ErrorNumber,
ERROR_MESSAGE() as ErrorMessage,
ERROR_PROCEDURE() as ErrorProcedure,
ERROR_STATE() as ErrorState,
ERROR_LINE() as ErrorLine
END CATCH
END
END
Exec spbasam_ProductSales 1,10

Answer 1

If the update fails because of a SQL error, then an error will be raised and it will jump to the catch block. However, if the update fails because it the SQL simply doesn't find a record to update, then no error will be raise and the next code will continue to execute. So, you should add a check to see how many rows were updated, right after the update statement, and raise your own error if nothing was updated.
...
UPDATE dbo.tblProduct set QtyAvailable = QtyAvailable - #QtyNeeded
WHERE ProductId = #ProductId and QtyAvailable >= #QtyNeeded
if ##rowcount = 0
raiserror('Insufficient quantity available.',16,1)
...
This will kick execution to the catch block if no records were updated.
And, per #JacobH suggestion in the comment to the original post, it would be good to NEVER allow the available quantity to go negative. So, if you add an additional check in the where clause, you could avoid going negative, and one user who orders at the same time as another won't be able to complete their order because the quantity will have decremented before their order is processed. (Good catch Jacob!)
Really, you could rewrite the whole thing to eliminate the variable #productavailabiltycount since you can simply check this inside the transaction, and only update the record if there is enough quantity available for the current transaction.