MySQL-左侧联接表中的MAX

Question

I have hit a brick wall on this one, hopefully someone smarter than myself could point me in the right direction?

I have an ORDERS table containing orders (surprisingly).

I also have a DOWNLOADS table. DOWNLOADS contains an ORDERID field, which is the ID of the order table.

DOWNLOADS has an ATTEMPT field. Each download creates a record in DOWNLOADS, and each record increments the value in ATTEMPT.

I just want to get the MAX value in ATTEMPT, so I can tell users what attempt they are on (or how many attempts they have left).

DOWNLOADS may or may not have any associated records in it, depending on if the user has not yet tried to download. Each download attempt, a record is created in DOWNLOADS.

So in simplistic terms I am trying to do something like:

SELECT orders.*,MAX(downloads.attempt)
FROM orders
LEFT JOIN downloads on downloads.orderid = orders.id
WHERE orders.userid = '123'

Or rather, I'm trying to get all user orders, with the MAX of download attempts where any download attempts (records in DOWNLOADS) exists - or null if there are none.

I've been going through stackoverflow all day looking at similar examples but somehow all similar examples are massively complicated. Is there a simple (or at least elegant) way of doing this?

A bit of help would be hugely appreciated.

Thanks.

Answer 1

Try this -

SELECT orders.userid, MAX(downloads.attempt)
FROM orders
LEFT JOIN downloads on downloads.orderid = orders.id
group by orders.userid

Answer 2

SELECT orders.*,MAX(downloads.attempt) FROM orders 
LEFT OUTER JOIN downloads on downloads.orderid = orders.id 
WHERE orders.userid = '123' 
GROUP BY orders.id

Answer 3

Try this:

SELECT orders.userid,MAX(IFNULL(downloads.attempt,0))
FROM orders
LEFT JOIN downloads on downloads.orderid = orders.id
GROUP BY orders.userid