Question

I have a table with sequential numbers (think invoice numbers or student IDs).

At some point, the user needs to request the previous number (in order to calculate the next number). Once the user knows the current number, they need to generate the next number and add it to the table.

My worry is that two users will be able to erroneously generate two identical numbers due to concurrent access.

I’ve heard of stored procedures, and I know that that might be one solution. Is there a best-practice here, to avoid concurrency issues?

Edit: Here’s what I have so far:

USE [master]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[sp_GetNextOrderNumber]
AS
BEGIN
    BEGIN TRAN

DECLARE @recentYear INT
DECLARE @recentMonth INT
DECLARE @recentSequenceNum INT

-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

    -- get the most recent numbers
SELECT @recentYear = Year, @recentMonth = Month, @recentSequenceNum = OrderSequenceNumber
FROM dbo.OrderNumbers 
WITH (XLOCK)
WHERE Id = (SELECT MAX(Id) FROM dbo.OrderNumbers)

    // increment the numbers
IF (YEAR(getDate()) > IsNull(@recentYear,0))
    BEGIN
        SET @recentYear = YEAR(getDate());
        SET @recentMonth = MONTH(getDate());
        SET @recentSequenceNum = 0;
    END
ELSE
    BEGIN
        IF (MONTH(getDate()) > IsNull(@recentMonth,0))
            BEGIN
                SET @recentMonth = MONTH(getDate());
                SET @recentSequenceNum = 0;
            END
        ELSE
            SET @recentSequenceNum = @recentSequenceNum + 1;
    END 

-- insert the new numbers as a new record   
INSERT INTO dbo.OrderNumbers(Year, Month, OrderSequenceNumber)
VALUES (@recentYear, @recentMonth, @recentSequenceNum)

COMMIT TRAN
END

This seems to work, and gives me the values I want. So far, I have not yet added any locking to prevent concurrent access.

Edit 2: Added WITH(XLOCK) to lock the table until the transaction completes. I’m not going for performance here. As long as I don’t get duplicate entries added, and deadlocks don’t happen, this should work.

Answer 1

you know that SQL Server does that for you, right? You can you a identity column if you need sequential number or a calculated column if you need to calculate the new value based on another one.

But, if that doesn’t solve your problem, or if you need to do a complicated calculation to generate your new number that cant be done in a simple insert, I suggest writing a stored procedure that locks the table, gets the last value, generate the new one, inserts it and then unlocks the table.

Read this link to learn about transaction isolation level

just make sure to keep the “locking” period as small as possible