Question

SQL Server 2008: I’ve got a situation here in which I wish to read from a table and write a row under certain conditions. The problem is that I don’t want another request coming it at exactly the same time and doing the same thing. I’ll try to explain here:

Table Name: RequestQueue

Columns:

RequestID, StartDate, EndDate, RequestResult

Sample Data

1, 12/4/10 1:00pm, 12/4/10 1:02pm, Success
2, 12/4/10 1:04pm, 12/4/10 1:05pm, Success
3, 12/4/10 1:00pm, NULL, NULL

When a page loads in my app, I want it to look at this table and if there is a request still pending like (ID #3) it will not do anything. Otherwise, if there are no requests pending, it creates a new row with the ID and StartDate filled in.

The issue is that we could get into a situation where the page is loaded twice at almost exactly the same time. If they happen to both read from the table before the new row is produced, then I could get two new rows in there. I want to have some sort of query that reads from the table and if there are no requests pending, inserts the new row with the StartDate filled in. I want that query to run all the way before another page can even read from this table so I don’t get the “double row” effect.

I might need “locking” or something, I googled that but haven’t found something for my exact situation. I’m sure this could be a simple stored procedure I just need a push in the right direction here.

Thanks,

Robert

Answer 1

Assuming that you just want to block all concurrent access to the table you could just do.

BEGIN TRAN 

DECLARE @StartDate datetime,
        @EndDate datetime

SELECT TOP 1
            @StartDate = StartDate,
            @EndDate = EndDate
        FROM     RequestQueue WITH(TABLOCK,XLOCK)
        ORDER BY RequestID DESC


IF @EndDate IS NULL
  SELECT @StartDate AS 'StartDate'
ELSE
  INSERT INTO RequestQueue (StartDate) 
  OUTPUT INSERTED.* /* Or use SCOPE_IDENTITY() instead*/
  VALUES (GETDATE())

COMMIT

Alternatively you could just serialise access to the SELECT/INSERT code inside the specific procedure without taking an exclusive table lock by using sp_getapplock