Question

I am saving documents to database, each document has to have an id with the format YYYY-00000:

• the first 4 characters are the current year
• the second five characters are numbers. They start with 1 each year and then increment.

For example I could have these documents in my database: 2011-00001, 2011-00002, 2011-00003, 2012-00001, 2012-00002, …

I am thinking something like this:

• add two columns to table Documents (Year and Number)
• Year is computed column, something like year(getdate())
• Number is computed column, which gets value from a function GetNextNumberForCurrentYear
• GetNextNumberForCurrentYear returns next number for the current year (for example select max(Number) + 1 from Documents where Year = year(getdate()), and some isnull checking)

But i fear, that two users could want to save the document at the same time and that they would receive the same Number. Is this possible? Any better ideas?

It is a ASP.NET C# web application, .NET 4.0, MSSQL 2005, I have the control over all the parts of the application.

PS: after insert I would like to return the Id of the new document to the user, so I would probably have to do something like: select Id from Documents where SomeId = scope_identity(), so I guess there should be an identity column somewhere…?

Edit (final solution): I get the next number from stored procedure, build the Id of the document (in format YYYY-00001) in .NET, save the whole document to the database (using TransactionScope for whole process) and then return the Id to the user.

create table DocumentNumbers ([Year] int not null, Number int not null default 1)
insert into DocumentNumbers ([Year], Number)
select 2012, 1 -- and more...

create procedure GetNextDocumentNumber
    @year int
as
begin
    declare @myResult table (nextNumber int) 

    update DocumentNumbers 
    set Number = isnull(Number, 0) + 1 
    output inserted.Number into @myResult 
    where [Year] = @year

    select top 1 nextNumber from @myResult
end

Answer 1

You could create a table NumberSeries, which contains a column Year and a column CurrentNo and a function that returns the next number from it like the following:

DECLARE @myResult TABLE (nextNumber INT)

UPDATE NumberSeries
OUTPUT INSERTED.NextNo INTO @myResult
SET
    CurrentNo = ISNULL(CurrentNo, 0) + 1
WHERE
    Year = Year(GetDate())

DECLARE @result INT
@result = (SELECT TOP 1 nextNumber FROM @myResult)

RETURN @result

This updates the NumberSeries table atomically and inserts the new value into the @myResult table variable. After that, it returns the first (and only) value from the @myResult table variable.

Everything else, like SCOPE_IDENTITY() and such may cause errors when using triggers or in other cases – the solution using the OUTPUT clause is safe.

EDIT
As for returning the ID of the inserted document: this is basically the same thing.

DECLARE @myDocId TABLE (yr int, no int)

INSERT INTO Documents
OUTPUT INSERTED.Year , INSERTED.YearID INTO @myDocID
...

SELECT TOP 1
    CAST(yr AS NVARCHAR) + 
    '_' + 
    RIGHT(REPLICATE('0', 5) + CAST(no AS NVARCHAR), 5) AS NewID