Find Logic Flaw, Order Number Generator

This procedure has been returning duplicate numbers. (Tested with scripts that called this proc and put value in a table.)

How can it return duplicates? Does the transaction Begin/Commit not guarantee transactional consistency?


CREATE PROCEDURE sp_UpdateOrderNumber @customer int AS
DECLARE @NewOrderId int,
@nSQLError int,
@nRowCount int
BEGIN TRAN
UPDATE CUSTOMERS
SET ORDER_NUMBER=ORDER_NUMBER + 1
WHERE COMPANY_ID=@customer
SELECT@nSQLError = @@error,
@nRowCount = @@rowcount
If @nSQLError != 0 OR @nRowCount != 1 /* Check for Errors */
Begin
Rollback Tran
Return -999
End
SELECTORDER_NUMBER
FROMCUSTOMERS
WHERECOMPANY_ID=@customer
SELECT@nSQLError = @@error,
@nRowCount = @@rowcount
If @nSQLError != 0 OR @nRowCount != 1 /* Check for Errors */
Begin
Rollback Tran
Return -998
End
COMMIT TRAN