Query to insert row after a specific amount of time

I am working on customized shopping cart application.

It have a hits table:

id            int
product_id    int
hit_time      datetime
session_id    varchar

Now when someone views a product, I record a hit.

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I need a query such that, record should be inserted only if hit_time has a difference of 30 mins (for same product_id, and session_id)

For example:

When recording first hit

id             product_id             hit_time                session_id
------------------------------------------------------------------------
1                  1                  2011-01-01 06:30:00      abcxyzmno
2                  5                  2011-01-01 06:32:00      abcxyzmno

Now, if visitor with same session_id, visit product page with product_id =1 in less than 30 mins, then data should not be inserted

So below row should not be inserted (because there is difference of 5 mins only)

3                  1                  2011-01-01 06:35:00      abcxyzmno

But if visitor visit after 30 mins, for same product, it should be recorded

3                  1                  2011-01-01 07:01:15      abcxyzmno

I want single insert query

I DON'T want 2 query, first to select max-time from table, and then insert it

Any help?

---

Are you looking for something like this?

INSERT INTO hits (product_id, hit_time, session_id)
SELECT $product_id, current_timestamp, $session_id
FROM hits
WHERE NOT EXISTS (
        SELECT id
        FROM hits
        WHERE product_id = $product_id 
          AND session_id = $session_id
   )
   OR (
            product_id = $product_id
        AND session_id = $session_id
        AND hit_time   < current_timestamp - 30*60
   )
LIMIT 1

Where $product_id and $session_id are the product and session IDs.

The NOT EXISTS part takes care of the case where (product_id,session_id) hasn't been recorded yet and the second part takes care of the case where (product_id,session_id) is there but it was longer than 30 minutes ago. And, of course, the LIMIT 1 in case (product_id,session_id) is there and older than 30 minutes ago and appears multiple times.