Question

How do I join a row from one table to a row with the MAX value for a given column on another table?

For example, I have a auctions table and an auction_bids table. I want to join the auctions table with the highest bid for that auction (i.e. highest value for column bid_amount AND where auction_id = x) in the auction_bids table.

Answer 1

It’s annoyingly complicated. You’d be better off with a “winner” flag in each winning auction_bid.

SELECT * FROM auctions a
INNER JOIN 
(
    /* now get just the winning rows */
    SELECT * FROM auction_bids x
    INNER JOIN
    (
        /* how to tell the winners */
        SELECT auction_id, MAX(bid_amount) as winner
        FROM auction_bids
        GROUP BY auction_id
    ) y
    ON x.auction_id = y.auction_id
    AND x.bid_amount = y.winner
) b
ON a.auction_id = b.auction_id

Note that auctions with zero bids will not be listed at all, and auctions with ties (can that happen?) will appear once for each tied bid.