Question

I have this stored procedure

CREATE PROCEDURE spGrabSerial
  @serial nvarchar(16) output
AS
BEGIN
  SET NOCOUNT ON;
  set @serial = (SELECT top 1 serial from tblSerial)
  update tblSerial set InUse = 1 where serial = @serial
END

How can I make sure that no other procedure grabs the same serial in between the select and the update?

Answer 1

Assuming SQL Server 2005+ you can use the OUTPUT clause to do it in one atomic operation (see Using tables as Queues).

 ;with cte as (
    select top(1) 
      serial, InUse
    from tblSerial with (rowlock, readpast)
    where InUse <> 1
    order by serial
   )
 update cte 
 set InUse = 1 
 output inserted.serial

Edit Just been reminded of a way of doing this that can assign directly to the output parameter without using the OUTPUT clause at all.

 with cte as (
    select top(1) 
      serial, InUse
    from tblSerial with (rowlock, readpast)
    where InUse <> 1
    order by serial
   )
 update cte 
 set InUse = 1, @serial = serial