How to create a queue like structure in SQL Server - sql

Question

Is there a good way to create a queue like structure in SQL Server?
Requirements:
When I insert rows, I want them to default to the bottom of the queue
When I select rows, I want to easily be able to get the top of the queue
Here's the tough one: I want to be able to easily move something up the queue, and reorient the rest. Example: move item 5 up to number 1, then 1-4 becomes 2-5
A simple identity column would work for requirements 1 and 2, but how would I handle 3?
Solution
I ended up implementing the solution from #roger-wolf
One difference, I used a trigger rather than a stored procedure to renumber. Here's my trigger code:
CREATE TRIGGER [dbo].[TR_Queue]
ON [dbo].[Queue]
AFTER INSERT, DELETE, UPDATE
AS
BEGIN
SET NOCOUNT ON;
-- Get the current max value in priority
DECLARE #maxPriority INT = COALESCE((SELECT MAX([priority]) FROM [dbo].[Queue]), 0);
WITH newValues AS (
-- Renumber by priority, starting at 1
SELECT [queueID]
,ROW_NUMBER() OVER(ORDER BY [priority] ASC) AS [priority]
FROM (
-- Pretend all nulls are greater than previous max priority
SELECT [queueID]
,COALESCE([priority], #maxPriority+1) AS [priority]
FROM [dbo].[Queue]
) AS tbl
)
UPDATE q
SET q.[priority] = newValues.[priority]
FROM [dbo].[Queue] AS qroger-wolf
INNER JOIN newValues
ON q.[queueID] = newValues.[queueID]
END
This works well for me as the queue is always relatively small and infrequently updated, so I don't have to work about performance of the trigger.

Answer 1

Use a float column for prioritisation and an approach similar to Celko trees:
If you have items with priorities 1, 2, and 3 and the last needs to become second, calculate an average between its new neighbours, 1.5 in this example;
If another one needs to become second, its priority would be 1.25. This can go on for quite a while;
When displaying queued items by their priority, use row_number() instead of float values in UI;
If items become too close together (say, 1e-10 or less), have a stored procedure ready to renumber them as integers.
The only deficiency I see here is that it becomes a bit more difficult to find N-th item in a middle of a queue, when it's neither first nor last. If you don't need that, the approach should work.

Answer 2

You could add a Priority column of type DateTime, and when you set a row as a priority row you set the current date-time in the Priority column and then use that as part of your order by criteria?

Answer 3

I had a similar requirement in a past project, what I did (and it worked):
Add column update_at_utc of type datetime2
When inserting, set update_at_utc = GETDATEUTC()
When retrieving, order by update_at_utc
When moving a row in the queue, for example between rows 3 and 4, simply take average of update_at_utc of these rows and use it to set update_at_utc of the row being moved.
Note 1: Point 4 assumes that the frequency of inserts and of moving the rows up/down the queue is such that datetime2 type has sufficient resolution. For example, if you insert 2 rows 1 millisecond apart, and then try to move 1000 rows between these 2 rows, then datetime2 resolution will be insufficient (https://learn.microsoft.com/en-us/sql/t-sql/data-types/datetime2-transact-sql?view=sql-server-2017). In such case, the moving of rows up/down the queue would need to be more complicated; When moving a row N places lower down:
Remember update_at_utc of the row N places lower down
For all rows between the current and the new position: assign row's update_at_utc to the preceding row's update_at_utc
Assign update_at_utc of the row being moved to the date remembered in point 1 above.
Note 2: I suggest UTC dates instead of local dates to avoid issues during a daylight saving switch.