Question

Our application needs a simple scheduling mechanism – we can schedule only one visit per room for the same time interval (but one visit can be using one or more rooms). Using SQL Server 2005, sample procedure could look like this:

CREATE PROCEDURE CreateVisit
    @start datetime, @end datetime, @roomID int
AS
BEGIN
DECLARE @isFreeRoom INT

BEGIN TRANSACTION

SELECT @isFreeRoom = COUNT(*) 
FROM visits V
INNER JOIN visits_rooms VR on VR.VisitID = V.ID
WHERE @start = start AND @end = [end] AND VR.RoomID = @roomID

IF (@isFreeRoom = 0)
BEGIN
    INSERT INTO visits (start, [end]) VALUES (@start, @end)
    INSERT INTO visits_rooms (visitID, roomID) VALUES (SCOPE_IDENTITY(), @roomID)
END

COMMIT TRANSACTION
END

In order to not have the same room scheduled for two visits at the same time, how should we handle this problem in procedure? Should we use SERIALIZABLE transaction isolation level or maybe use table hints (locks)? Which one is better?

Answer 1

I would have the calling application pass in a comma separated list of room IDs and split them in the SQL, inserting all rows with one INSERT. Doing that, with the proper locking hints (on a single SELECT), should allow your scheduling procedure to work.

I prefer the number table approach to split a string in TSQL, but you can use your own split method if you have one. Here is how to make the number table split approach work:

For this method to work, you need to do this one time table setup:

SELECT TOP 10000 IDENTITY(int,1,1) AS Number
    INTO Numbers
    FROM sys.objects s1
    CROSS JOIN sys.objects s2
ALTER TABLE Numbers ADD CONSTRAINT PK_Numbers PRIMARY KEY CLUSTERED (Number)

Once the Numbers table is set up, create this split function:

CREATE FUNCTION [dbo].[FN_ListToTable]
(
     @SplitOn  char(1)      --REQUIRED, the character to split the @List string on
    ,@List     varchar(8000)--REQUIRED, the list to split apart
)
RETURNS TABLE
AS
RETURN 
(

    ----------------
    --SINGLE QUERY-- --this will not return empty rows
    ----------------
    SELECT
        ListValue
        FROM (SELECT
                  LTRIM(RTRIM(SUBSTRING(List2, number+1, CHARINDEX(@SplitOn, List2, number+1)-number - 1))) AS ListValue
                  FROM (
                           SELECT @SplitOn + @List + @SplitOn AS List2
                       ) AS dt
                      INNER JOIN Numbers n ON n.Number < LEN(dt.List2)
                  WHERE SUBSTRING(List2, number, 1) = @SplitOn
             ) dt2
        WHERE ListValue IS NOT NULL AND ListValue!=''

);
GO 

You can now easily split a CSV string into a table and join on it:

select * from dbo.FN_ListToTable(',','1,2,3,,,4,5,6777,,,')

OUTPUT:

ListValue
-----------------------
1
2
3
4
5
6777

(6 row(s) affected)

This is what I would then make your procedure:

CREATE PROCEDURE CreateVisit
    @start datetime, @end datetime, @roomIDs varchar(8000)
AS
BEGIN
DECLARE @RowID INT

BEGIN TRANSACTION

IF NOT EXISTS (SELECT
                   1
                   FROM visits_rooms           (HOLDLOCK,UPDLOCK)  v 
                       INNER JOIN dbo.FN_ListToTable(',',@roomIDs) r ON v.RoomID=r.ListValue
                   WHERE @start = start AND @end = [end] AND VR.RoomID = @roomID --copy of your logic, but shouldn't it be WHERE start>=@start AND [end]<=@end
              )
BEGIN
    INSERT INTO visits (start, [end]) VALUES (@start, @end)
    SELECT @RowID=SCOPE_IDENTITY()
    INSERT INTO visits_rooms
            (visitID, roomID)
        SELECT 
            @RowID, r.ListValue
            FROM dbo.FN_ListToTable(',',@roomIDs) r

END

COMMIT TRANSACTION
END

GO

if you have many RoomIDs one one schedule attempt, you could split them into a @TempTable variable or an actual table #TempTable first and then reuse it in the IF EXISTS and INSERT SELECT.