SQL Need advice how to add timestamp to this query - sql

Question

I have this code:
select Users.phoneMac, Users.apMac, Locations.Lon, Locations.Lat
from Locations, Users
inner join (
select u.phoneMac, max(u.strenght) as most
from Users u, Locations l
where u.apMac = l.apMac
group by u.phoneMac
) as ij on ij.phoneMac=Users.phoneMac and Users.strenght = ij.most
where Locations.apMac = Users.apMac;
It worked for me fine but when I added more data to users table this query calculated results from all the data and I wanted to get results just from latest data. So I added timestamp to Users table.
So can you help me fix this code so it first take only data from latest timestamp for every user(users.phoneMac)(there can be more then 1 row of data for same phoneMac) and then do the rest of calculations.

Answer 1

You're already picking the max value of the "strenght" field and joining on that, so why not use the same approach again for your timestamp field? Something like:
SELECT Users.phoneMac, Users.apMac, Locations.Lon, Locations.Lat
FROM Locations
INNER JOIN Users
ON Users.apMac = Locations.apMac
INNER JOIN (
SELECT u.phoneMac, max(u.strenght) AS most
FROM Locations l
INNER JOIN Users u ON u.apMac = l.apMac
GROUP BY u.phoneMac) AS ij
ON ij.phoneMac = Users.phoneMac
AND Users.strenght = ij.most
INNER JOIN (
SELECT u2.phoneMac, max(u2.timestampfield) AS latest
FROM Locations l2
INNER JOIN Users u2 ON u2.apMac = l2.apMac
GROUP BY u2.phoneMac) AS ijk
ON ijk.phoneMac = Users.phoneMac
AND Users.timestampfield = ij.latest;
(By the way, using the old join syntax with comma and the WHERE clause makes it harder to understand the logic, and occasionally makes the logic wrong. The new join syntax with ON is really a lot better.)