Is there a way to SELECT and UPDATE rows at the same time?

Question

I'd like to update a set of rows based on a simple criteria and get the list of PKs that were changed. I thought I could just do something like this but am worried about possible concurrency problems:

SELECT Id FROM Table1 WHERE AlertDate IS NULL;
UPDATE Table1 SET AlertDate = getutcdate() WHERE AlertDate IS NULL;

If that is wrapped in a transaction are there any concurrency issues that can occur? Or is there a better way to do this?

Answer 1

It'd be easier to do your UPDATE first and then run 'SELECT ID FROM INSERTED'.

Take a look at SQL Tips for more info and examples.

Answer 2

Consider looking at the OUTPUT clause capability of UPDATE.

http://msdn.microsoft.com/en-us/library/ms177564(SQL.90).aspx

Answer 3

if it's inside the transaction, the database locking system will take care of concurrency issues. of course, if you use one (the mssql default is that it uses lock, so it states if you don't override that)

Answer 4

Edit: my bad, you wanted the select to show results after the update, not update from a select.

Have you tried a sub-select?

SQL> update mytable set mydate = sysdate where mydate in (select mydate from mytable where mydate is null);

Answer 5

One way to handle this is to do it in a transaction, and make your SELECT query take an update lock on the rows selected until the transaction completes.

BEGIN TRAN

SELECT Id FROM Table1 WITH (UPDLOCK)
WHERE AlertDate IS NULL;

UPDATE Table1 SET AlertDate = getutcdate() 
WHERE AlertDate IS NULL;

COMMIT TRAN

This eliminates the possibility that a concurrent client updates the rows selected in the moment between your SELECT and your UPDATE.

When you commit the transaction, the update locks will be released.

Another way to handle this is to declare a cursor for your SELECT with the FOR UPDATE option. Then UPDATE WHERE CURRENT OF CURSOR. The following is not tested, but should give you the basic idea:

DECLARE cur1 CURSOR FOR
  SELECT AlertDate FROM Table1 
  WHERE AlertDate IS NULL
  FOR UPDATE;

DECLARE @UpdateTime DATETIME

SET @UpdateTime = GETUTCDATE()

OPEN cur1;

FETCH NEXT FROM cur1;

WHILE @@FETCH_STATUS = 0
BEGIN

  UPDATE Table1 AlertDate = @UpdateTime
  WHERE CURRENT OF cur1;

  FETCH NEXT FROM cur1;

END

Answer 6

Perhaps something more like this?

declare @UpdateTime datetime

set @UpdateTime = getutcdate()

update Table1 set AlertDate = @UpdateTime where AlertDate is null

select ID from Table1 where AlertDate = @UpdateTime

Answer 7

in SQL 2008 a new TSQL statement "MERGE" is introduced which performs insert, update, or delete operations on a target table based on the results of a join with a source table. You can synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.

http://blogs.msdn.com/ajaiman/archive/2008/06/25/tsql-merge-statement-sql-2008.aspx http://msdn.microsoft.com/en-us/library/bb510625.aspx