选择具有MAX（时间戳）的行

Question

In our database we have addresses, services and missions. Simplified, our data looks like this:

Adress:
id
name

Service:
id
Address_id

Mission:
id
start
Service_id

What I tried to achieve is getting the Mission.id of the Mission that started last for each address. Every address can be in multiple services and each service can have multiple missions. I tried many different ways, but none of which worked. My latest attempt:

SELECT
  a.id AS addressID,
  m.id AS missionID
FROM Address AS a
JOIN Service AS s ON s.Address_id = a.id
JOIN Mission AS m ON m.Service_id = s.id AND m.start = (SELECT MAX(start) FROM Mission WHERE start < ".time().")

The problem is, that multiple missions start at the same time, so I'm randomly joining any mission with the the correct start, but it's not guaranteed that it really belongs to my address. Any tips / inputs are highly welcome.

Answer 1

I think you want a correlated subquery, for the approach you are using. I think the logic is:

SELECT a.id AS addressID, m.id AS missionID
FROM Address a JOIN
     Service s
     ON s.Address_id = a.id JOIN
     Mission m
     ON m.Service_id = s.id
WHERE m.start = (SELECT MAX(m2.start)
                 FROM Mission m2 JOIN
                      Service s2
                      ON m2.Service_Id = s2.id
                 WHERE s2.Address_Id = s.AddressId AND
                       m2.start < ".time()."
                );