How to set up a lock inside a store procedure? - sql

Question

I am trying to set up a read/write lock in SQL Server. My stored procedure is
CREATE PROCEDURE test()
AS
BEGIN
SELECT VALUE FROM MYTABLE WHERE ID=1
UPDATE MYTABLE SET VALUE=VALUE+1 WHERE ID=1
END
I would like to be sure tha no-one else is going to read or update the "Value" field while this stored procedure is being executed.
I read lots of posts and I read that in SQL Server should be enough to set up a transaction.
CREATE PROCEDURE test()
AS
BEGIN
BEGIN TRANSACTION
SELECT VALUE FROM MYTABLE WHERE ID=1
UPDATE MYTABLE SET VALUE=VALUE+1 WHERE ID=1
COMMIT TRANSACTION
END
But to me this is not enough, since I tried launching two parallel connections, both of them using this stored procedure. With SQL Server Management Studio's debugger, i stopped the first execution inside the transaction, and i observed that the second transaction has been executed!
So i tried to add ISOLATION LEVEL
CREATE PROCEDURE test()
AS
BEGIN
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
BEGIN TRANSACTION
SELECT VALUE FROM MYTABLE WHERE ID=1
UPDATE MYTABLE SET VALUE=VALUE+1 WHERE ID=1
COMMIT TRANSACTION
END
but the result is the same.
I also tried to set isolation level in the client code
SET TRANSACTION ISOLATION LEVEL READ COMMITTED
EXEC test
but again nothing changed.
My question is: in such situation, which is the correct way to set up a lock that blocks the others?
thank you

Answer 1

in such situation, which is the correct way to set up a lock that blocks the others?
The correct lock here is to read the table with an UPDLOCK, in a transaction.
SELECT VALUE FROM MYTABLE with (UPDLOCK)
WHERE ID=1
You can also use an OUTPUT clause to update and return the value in a single query, which will also prevent two sessions from reading and updating the same value
update MyTable set value = value+1
output inserted.value
However you should not generate keys like this. Only one session can generate a key at a time, and the locking to generate the key is held until the end of the session's current transaction. Instead use a SEQUENCE or an IDENTITY column.