Return unlocked rows in a “select top n” query

Question

I need to have a MsSql database table and another 8 (identical) processes accessing the same table in parallel - making a select top n, processing those n rows, and updating

Answer 1

Here's a sample I blogged about a while ago:

The READPAST hint is what ensures multiple processes don't block each other when polling for records to process. Plus, in this example I have a bit field to physically "lock" a record - could be a datetime if needed.

DECLARE @NextId INTEGER
BEGIN TRANSACTION

-- Find next available item available
SELECT TOP 1 @NextId = ID
FROM QueueTable WITH (UPDLOCK, READPAST)
WHERE IsBeingProcessed = 0
ORDER BY ID ASC

-- If found, flag it to prevent being picked up again
IF (@NextId IS NOT NULL)
    BEGIN
        UPDATE QueueTable
        SET IsBeingProcessed = 1
        WHERE ID = @NextId
    END

COMMIT TRANSACTION

-- Now return the queue item, if we have one
IF (@NextId IS NOT NULL)
    SELECT * FROM QueueTable WHERE ID = @NextId