如何为此SQL获取正确的锁？

Question

My database is SQL Server 2005/8. In a booking system we have a limit of 24 bookings on an event. This code in a stored procedure checks: - that the current user (@UserId) is not already booked on the event (@EventsID) - that the current event has a current booking list of under 24 - inserts a new booking.

BEGIN TRANSACTION 
IF (((select count (*) from dbo.aspnet_UsersEvents with (updlock) 
      where UserId = @UserId and EventsId = @EventsId) = 0) 
AND  ((SELECT Count(*)  FROM dbo.aspnet_UsersEvents with (updlock) 
      WHERE EventsId = @EventsId) < 24))
BEGIN
  insert into dbo.aspnet_UsersEvents (UserId, EventsId) 
      Values (@UserId, @EventsId)
END
COMMIT

The problem is that it is not safe. Two users might perform the test simultaneously and conclude they can both book. Both insert a row and we end up with 25 bookings.

Simply enclosing it in a transaction does not work. I tried adding WITH (UPDLOCK) to the selects in the hope that one would take update locks and keep the other out. That does not work.

Answer 1

Three options:

1. SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
2. Change the lock hint to WITH (UPDLOCK, HOLDLOCK)
3. Add a unique constraint to dbo.aspnet_UsersEvents and a TRY/CATCH around the insert.

You can use the following script to confirm that the lock is taken and immediately released if you omit HOLDLOCK . You will also see that the lock is not released (no 'releasing lock reference on KEY' output) when HOLDLOCK is used.

( Gist script )

Answer 2

Just do it in one statement, at READ COMMITTED or higher.

INSERT dbo.aspnet_UsersEvents
       (UserId,EventsId)
OUTPUT inserted.UserEventsId -- Or whatever, just getting back one row identifies the insert was successful
SELECT @UserId
       , @EventsId
 WHERE ( SELECT COUNT (*)
           FROM dbo.aspnet_UsersEvents
          WHERE UserId = @UserId
                AND EventsId = @EventsId ) = 0
       AND ( SELECT COUNT(*)
               FROM dbo.aspnet_UsersEvents
              WHERE EventsId = @EventsId ) < 24 

_{Side note: your SELECT COUNT(*) for duplicate checking seems excessive, personally I'd use NOT EXISTS(SELECT NULL FROM ... WHERE UserID = ..., EventsID = ... .}