Save sequence in special table or get last value? - sql-server

Question

Do you have advice or best practice or recommendation about sequence for identifiers?
I work on a database where all the identifier or document numbers are 'complex' sequence. For example the sequence for our invoices are INVCCC-2016-0000 where INV is fixed, CCC is the client reference, 2016 is the year and 0000 is a counter from 1 to 9999. This number must be unique and at this moment we keep it in a columns.
When I create a new invoice I need to check the last number created for this client this year then increment it of one then save my data in my database.
I see two way to do it
I create a special table that contain and maintain all last used number for each client. Each time I have a new invoice I check the number in this table, I increment it of one, I use this number to save my invoice then I update the sequence table. 1 READ, 1 INSERT, 1 UPDATE (may be an INSERT in sequence is new)
var keyType = "INV" + ClientPrefix + "-" + Year;
var keyValue = Context.SequenceTable.SingleOrDefault(y => y.KeyType == keyType).KeyValue;
I check the last number in my invoice table, I increment it then I save my invoice. 1 READ, 1 INSERT. I don't need tu update another table and this seems more logic to me. But my database administrator tell me this can create lock or other troubles.
var keyType = "INV" + ClientPrefix + "-" + Year;
var keyValue = Invoices.Where(y => y.InvoiceId.StartsWith(keyType)).OrderByDescending(y => y.InvoiceId).LastOrDefault();
Note I use SQL server before 2015 version and then before the SEQUENCE feature. I fear an inconsistency with solution 1. I fear performance issue with solution 2.

Answer 1

I suggest a table to maintain the last used value for the custom sequence. This method will also ensure there are no gaps, if that is a business requirement.
The example below uses a transactional stored procedure to avoid a race condition in the event an invoice number is generated concurrently for the same client. You'll need to change the CATCH block to use RAISERROR instead of THROW if you are using a pre SQL Server 2012 version.
CREATE TABLE InvoiceSequence (
ClientReferenceCode char(3) NOT NULL
, SequenceYear char(4) NOT NULL
, SequenceNumber smallint NOT NULL
CONSTRAINT PK_InvoiceSequence PRIMARY KEY (ClientReferenceCode, SequenceYear)
);
GO
CREATE PROC dbo.GetNextInvoiceSequence
#ClientReferenceCode char(3) = 'CCC'
, #SequenceYear char(4) = '2016'
AS
SET XACT_ABORT, NOCOUNT ON;
DECLARE #SequenceNumber smallint;
BEGIN TRY
BEGIN TRAN;
UPDATE dbo.InvoiceSequence
SET #SequenceNumber = SequenceNumber = SequenceNumber + 1
WHERE
ClientReferenceCode = #ClientReferenceCode
AND SequenceYear = #SequenceYear;
IF ##ROWCOUNT = 0
BEGIN
SET #SequenceNumber = 1;
INSERT INTO dbo.InvoiceSequence(ClientReferenceCode, SequenceYear, SequenceNumber)
VALUES (#ClientReferenceCode, #SequenceYear, #SequenceNumber);
END;
IF #SequenceNumber > 9999
BEGIN
RAISERROR('Invoice sequence limit reached for client %s year %s', 16, 1, #ClientReferenceCode, #SequenceYear) AS InvoiceNumber;
END;
COMMIT;
SELECT 'INV' + #ClientReferenceCode + '_' + #SequenceYear + '_' + RIGHT('000' + CAST(#SequenceNumber AS varchar(4)), 4);
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0 ROLLBACK;
THROW;
END CATCH;
GO
--sample usage:
--note that year could be assigned in the proc if it is based on the current date
EXEC dbo.GetNextInvoiceSequence
#ClientReferenceCode = 'CCC'
, #SequenceYear = '2016';