MYSQL: Select Then Update. Double Processing Issue

Question

The Issue:

I am building a google checkout callback interface. The issue I am having is that google sends 2 responses at the same time back to the site and the site processes both of them, so I end up with 2 updates. The code is written to check if the entry exists, and if it does not insert or update as needed.

The time stamps will be identical and it will post 2 transactions. I had the database at Innodb, but I changed it to MyISAM in hopes that it would resolve the issue.

Question:

Is it possible to have an SQL query select and then update on a conditional. Or is there something else I can do to get around this issue?

Answer 1

You can use EXIST to perform your update only of your select return a result

 UPDATE TABLE1 SET col=`value` 
 where exists (select 1 from TABLE2 where condition)

Answer 2

I think you need to clarify what you mean by "Google sends 2 responses at the same time".

I'm assuming you are referring to Google Checkout API - and if so, each notification Google sends represents "something". You have to identify what the Notification is - in other words, you maybe "fixing" an issue that doesn't exist and are doing so based on your perception of what Google sends (rather than noticing that they, each Notification, are distinct).

Hth....