如何在SQL Server中创建类似于结构的队列

Question

Is there a good way to create a queue like structure in SQL Server?

Requirements:

1. When I insert rows, I want them to default to the bottom of the queue
2. When I select rows, I want to easily be able to get the top of the queue
3. 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

您可以添加类型为DateTime的Priority列，然后将某行设置为优先级行时，在Priority列中设置当前日期时间，然后order by条件将其用作order by一部分？

Answer 3

I had a similar requirement in a past project, what I did (and it worked):

1. Add column update_at_utc of type datetime2
2. When inserting, set update_at_utc = GETDATEUTC()
3. When retrieving, order by update_at_utc
4. 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://docs.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:

1. Remember update_at_utc of the row N places lower down
2. For all rows between the current and the new position: assign row's update_at_utc to the preceding row's update_at_utc
3. 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.