Question

I have a table which contains header information for transactions. The transactions belong to different projects.

In the header I have columns:

rhguid - uniqueidentifier
rhserial - int
rh_projectID - int

First I insert the row (there’s more columns)

Then I calculate the serial number for that project:

update responseheader 
set rhSerial = 1 + (select isnull(max(rhSerial), 0) 
                    from responseheader 
                    where (rhstatus = 0) AND (rh_projectID = 1234)) 
where 
   (rhGUID = <preassignedGUID>);

However when there are many transactions happening at the same time for a project I am finding duplicate rhserial values.

I’m doing this in classic ASP with SQL Server 2008.

Is there a better way?

Answer 1

From your example, it doesn’t look like you’re using a transaction. My guess is that the SELECT portion of the statement is running as READ UNCOMMITTED, otherwise you would not see duplicates. There are ways to start transactions with ADO, but I prefer using stored procedures instead.

Try implementing something like this:

CREATE PROC dbo.ResponseHeader_Insert
  <more data to insert>,
  @ProjectID INT,
  @Status SMALLINT
as

insert responseheader (column names here)
select <param values here>, isnull(max(rhSerial), 0) + 1
from responseheader  
where (rhstatus = @Status) AND (rh_projectID = @ProjectID))  

If this doesn’t work for ya, try creating sequence tables (one for each sequence).

create table <tablename> (
      SeqID int identity(1,1) primary key,
      SeqVal varchar(1)
)

Create a procedure to get the next identity:

create procedure GetNewSeqVal_<tablename>
as
begin
      declare @NewSeqValue int

      set NOCOUNT ON

      insert into <tablename> (SeqVal) values ('a')
      set @NewSeqValue = scope_identity()
      delete from <tablename> WITH (READPAST)
return @NewSeqValue
end

If there are too many sequence tables that need to be created or you want to create sequences on the fly, try this approach:

Create table AllSequences (
      SeqName nvarchar(255) primary key, -- name of the sequence
      Seed int not null default(1), -- seed value
      Incr int not null default(1), -- incremental
      Currval int 
)
Go

create procedure usp_CreateNewSeq
      @SeqName nvarchar(255),
      @seed int = 0,
      @incr int = 1
as

begin

      declare @currval int
      if exists (
            select 1 from AllSequences
            where SeqName = @SeqName )

      begin
            print 'Sequence already exists.'
            return 1    
      end

      if @seed is null set @seed = 1
      if @incr is null set @incr = 1
      set @currval = @seed

      insert into AllSequences (SeqName, Seed, Incr, CurrVal)
      values (@SeqName, @Seed, @Incr, @CurrVal)
end
go


create procedure usp_GetNewSeqVal

      @SeqName nvarchar(255)
as

begin
      declare @NewSeqVal int
      set NOCOUNT ON

      update AllSequences
      set @NewSeqVal = CurrVal = CurrVal+Incr
      where SeqName = @SeqName

      if @@rowcount = 0 begin
            print 'Sequence does not exist'
            return
      end
      return @NewSeqVal
end
go