Question

I’m setting up a new database scheme for an auction website and trying to send email to users for when specific items they’re looking for are posted. I have three tables:

auctions

-each item that is posted by user gets an entry here with the title, a googleID, current bid, etc.

alerts

-these are setup by users and they include the googleID of the item they’re looking for, their user ID, an enabled boolean and a timestamp

notifications

-this keeps track of which users have been alerted about which items so that we don’t alert the same user about the same item every time our cron.php is run. Includes alert ID and the auction ID

So I’m trying to create a query that Selects the following:

• alert is enabled in alerts table
• googleID in alerts table matches googleID in auctions table
• auction is not closed in auctions table
• alert ID & auction ID in notifications table are not on the same row (this is the part I’m having trouble with. I need to make sure a single auction ID can have multiple alerts sent out for it, but never the same auction to any user multiple times)

This is the query I have so far, but I’m really stuck on that last part and I’ve looked everywhere I can think of for this. Hopefully I’ve explained myself enough, let me know if any clarification would help and thanks in advance!

$sql = "SELECT a.user, b.id, b.title, b.googleimg, b.authors
        FROM alerts a, auctions b, notifications c
        WHERE a.googleID = b.googleid AND a.enabled = 'y' AND b.closed = 0 AND ????";

Answer 1

How about –

SELECT
    alerts.user,
    auctions.id,
    auctions.title,
    auctions.googleimg,
    auctions.authors 
FROM alerts   
INNER JOIN auctions
    ON alerts.googleID = auctions.googleID
LEFT JOIN notifications
    ON alerts.alertID = notifications.alertID
    AND auction.auctionID = notifications.auctionID
WHERE alerts.enabled = 'y'
AND auction.closed = 0
AND notifications.alertID IS NULL