Question

I’m a long time Firebird user and it has a feature called Generators (I think Oracle also has it and it’s called Sequences). I’m new to SQL Server and I need to simulate the same feature. I can’t use an identity field to solve my problem. I need a named series of values, not a unique number for each row.

My biggest concern was about more than one user calling the procedure at the same time and getting duplicated values, so I decided to lock using the SERIALIZABLE isolation level.

So I came up with this code (I’m using SQL Server 2005):

CREATE TABLE dbo.GENERATORS     (       [NAME] VARCHAR(30) NOT NULL,       [VALUE] INT,       CONSTRAINT UNQ_GENERATORS_NAME UNIQUE NONCLUSTERED ( [NAME] )     ) GO  CREATE PROCEDURE GEN_ID     (       @GENERATOR_NAME VARCHAR(30)     ) AS      DECLARE @RETURN_VALUE INT ;     SET NOCOUNT ON     SET TRANSACTION ISOLATION LEVEL SERIALIZABLE     BEGIN TRANSACTION     UPDATE  GENERATORS     SET     [VALUE] = [VALUE] + 1     WHERE   [NAME] = @GENERATOR_NAME     IF @@ROWCOUNT = 0          BEGIN             INSERT  INTO dbo.GENERATORS ( [NAME], [VALUE] )             VALUES  ( @GENERATOR_NAME, 1 )             SET @RETURN_VALUE = 1         END     ELSE          BEGIN             SELECT  @RETURN_VALUE = [VALUE]             FROM    GENERATORS             WHERE   [NAME] = @GENERATOR_NAME         END     COMMIT TRANSACTION     RETURN @RETURN_VALUE GO 

So my questions are:

• Is this a good solution?
• Is there any better way?

Thanks.

Answer 1

Instead of UPDATE and SELECT, to get the value back that you just added, you can do

UPDATE  GENERATORS SET     @RETURN_VALUE = [VALUE] = [VALUE] + 1 WHERE   [NAME] = @GENERATOR_NAME 

which may remove the need for isolation level

I would prefer to return @RETURN_VALUE as an OUTPUT parameter, rather than as a Return value – leaving the Return Value free for any error code during execution.