Generate a unique column sequence value based on a query handling concurrency

Question

I have a requirement to automatically generate a column\'s value based on another query\'s result. Because this column value must be unique, I need to take into consideratio

Answer 1

One possibility:

Create a counter table:

create table Counter (
   Id int identify(1,1),
   Name varchar(64)
   Count1 int
)

Name is a unique identifier for the sequence, and in your case name would be CustomerName-Month-Year i.e. you would end up with a row in this table for every Customer/Year/Month combination.

Then write a stored procedure similar to the following to allocate a new sequence number:

CREATE PROCEDURE [dbo].[Counter_Next]
(
  @Name varchar(64)
  , @Value int out -- Value to be used
)
AS
BEGIN
  set nocount on;

  declare @Temp int;

  begin tran
    -- Ensure we have an exclusive lock before changing variables
    select top 1 1 from dbo.Counter with (tablockx);

    set @Value = null; -- if a value is passed in it stuffs us up, so null it

    -- Attempt an update and assignment in a single statement
    update dbo.[Counter] set
      @Value = Count1 = Count1 + 1
    where [Name] = @Name;

    if @@rowcount = 0 begin
      set @Value = 10001; -- Some starting value
      -- Create a new record if none exists
      insert into dbo.[Counter] ([Name], Count1)
        select @Name, @Value;
    end
  commit tran

  return 0;
END