Trying to query and then update a table in one transaction - sql

Question

Spec for the stored procedure is:
To select and return the Id from my table tb_r12028dxi_SandpitConsoleProofClient (order is not important just the top 1 found will do) and as soon as I've selected that record it needs to be marked 'P' so that it does not get selected again.
Here is the stored procedure:
ALTER PROCEDURE [dbo].[r12028dxi_SandpitConsoleProofSweep]
#myId INT OUTPUT
AS
/*
DECLARE #X INT
EXECUTE [xxx].[dbo].[r12028dxi_SandpitConsoleProofSweep] #X OUTPUT
SELECT #X
*/
DECLARE #NumQueue INT = (
SELECT [cnt] = COUNT(*)
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient
WHERE [Status] IS NULL
);
IF #NumQueue > 0
BEGIN
BEGIN TRANSACTION;
DECLARE #foundID INT = (SELECT TOP 1 Id FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient WHERE [Status] IS NULL);
UPDATE x
SET x.[Status] = 'P'
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient x
WHERE x.Id = #foundID
SET #myId = #foundID;
RETURN;
COMMIT TRANSACTION;
END;
GO
It is returning the error message:
Transaction count after EXECUTE indicates a mismatching number of
BEGIN and COMMIT statements. Previous count = 0, current count = 1.
I've just added the Update script and the BEGIN TRANSACTION; and COMMIT TRANSACTION; before that it worked fine when it looked like the following...
ALTER PROCEDURE [dbo].[r12028dxi_SandpitConsoleProofSweep]
#myId INT OUTPUT
AS
/*
DECLARE #X INT
EXECUTE [xxx].[dbo].[r12028dxi_SandpitConsoleProofSweep] #X OUTPUT
SELECT #X
*/
DECLARE #NumQueue INT = (
SELECT [cnt] = COUNT(*)
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient
WHERE [Status] IS NULL
);
IF #NumQueue > 0
BEGIN
SELECT TOP 1 #myId = Id FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient;
RETURN;
END;
GO
I added the BEGIN TRANSACTION; / COMMIT TRANSACTION; because I wanted to ensure that the data gets read into the output variable AND that the UPDATE happens. Should I just leave out this section of the procedure?

Answer 1

You have "RETURN;" before "COMMIT TRANSACTION;" which means "COMMIT TRANSACTION;" is never executed.

Answer 2

Give that you want:
and as soon as I've selected that record it needs to be marked 'P' so
that it does not get selected again.
you can achieve that in a single statment (and not in a transaction)
ALTER PROCEDURE [dbo].[r12028dxi_SandpitConsoleProofSweep]
#myId INT OUTPUT
AS
BEGIN
UPDATE x
SET x.[Status] = 'P',
#myID = x.ID
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient x
/* a sample join to get your single row in an update statement */
WHERE x.ID = (SELECT MIN(ID)
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient sub
WHERE ISNULL(sub.[Status], '') != 'P')
END
Note: When dealing with concurrent processing (ie: two threads trying to select from a single queue) it's more about the locking behavior than doing it inside a single transaction.

Answer 3

As an alternative to a perfectly reasonable suggestion by #Andrew Bickerton, you could also use a CTE and the ROW_NUMBER() function, like this:
WITH ranked AS (
SELECT
Id,
[Status],
rnk = ROW_NUMBER() OVER (ORDER BY Id)
FROM xxx.DBO.tb_r12028dxi_SandpitConsoleProofClient
WHERE [Status] IS NULL
)
UPDATE ranked
SET
[Status] = 'P',
#myId = Id
WHERE rnk = 1
;
The ROW_NUMBER() function assigns rankings to all rows where [Status] IS NULL, which allows you to update only a specific one.
The use of the CTE as the direct target of the UPDATE statement is absolutely legitimate in this case, as the CTE only pulls rows from one table. (This is similar to the use of views in UPDATE statements.)