Select multiple columns based on the max of another column

Question

I have and Oracle 11g Database with the following three tables (simplified):

IP Table, containing an IP identifier, the IP, and IP status and a FQDN. IPs might be repeated.

+-------+-------------+-----------+-----------+
| ID_IP |     IP      | IP_STATUS |   FQDN    |
+-------+-------------+-----------+-----------+
|     1 | 192.168.1.1 |         1 | test.com  |
|     2 | 192.168.1.1 |         2 | test.com  |
|     3 | 192.168.1.1 |         3 | test.com  |
|     4 | 10.10.45.12 |         2 | test2.com |
+-------+-------------+-----------+-----------+

VLAN Table, containing and VLAN identifier and the VLAN number

+---------+-------------+
| VLAN_ID | VLAN_NUMBER |
+---------+-------------+
|       1 |           3 |
|       2 |           5 |
|       3 |           7 |
+---------+-------------+

A Table correlating VLANs and IPs:

+-------+---------+
| IP_ID | VLAN_ID |
+-------+---------+
|     1 |       1 |
|     2 |       2 |
|     3 |       3 |
|     4 |       2 |
+-------+---------+

In the actual IP Table, the primary key is the tuple (IP, IP_STATUS). My goal is to create a new table eliminating the IP_STATUS, and to do that, I want to aggregate IPs and get the ID_IP and FQDN of the IP whose VLAN_NUMBER is higher. The answer for the SELECT query would be something like this:

+-------+-------------+-----------+
| ID_IP |     IP      |   FQDN    |
+-------+-------------+-----------+
|     3 | 192.168.1.1 | test.com  |
|     4 | 10.10.45.12 | test2.com |
+-------+-------------+-----------+

I can get the IP using the following query:

SELECT i.IP, max(v.VLAN_ID)
FROM IPS i 
LEFT JOIN VLAN_IP_REL v_i ON i.ID_IP=v_i.ID_IP
LEFT JOIN VLANS v ON v_i.ID_VLAN=v.ID_INSTANCIA
GROUP BY i.IP;

What I don't know is how to get the other columns. I tried with a subquery like the following:

SELECT i.ID_IP, i.IP, i.FQDN 
FROM IPS i
WHERE i.IP IN (
    SELECT i.IP, max(v.VLAN_ID)
    FROM IPS i 
    LEFT JOIN VLAN_IP_REL v_i ON i.ID_IP=v_i.ID_IP
    LEFT JOIN VLANS v ON v_i.ID_VLAN=v.ID_INSTANCIA
    GROUP BY i.IP;
)

But it doesn't work, since the subquery returns two values, and I need the max(vlan.VLAN_ID) to do the aggregation.

How could I get the right IP_ID?

Thank you!

Answer 1

You can use an analytical clause to split by IP and order by VLAN_NUMBER , then filter to retain only the first line in each group :

SELECT ID_IP, IP, FQDN
FROM (
    SELECT ROW_NUMBER() OVER (PARTITION BY i.IP ORDER BY v.VLAN_NUMBER DESC) AS NB,
     i.ID_IP, i.IP, i.FQDN
    FROM IPS i 
    LEFT JOIN VLAN_IP_REL v_i ON i.ID_IP = v_i.ID_IP
    LEFT JOIN VLANS v ON v_i.VLAN_ID = v.VLAN_ID
) t_a
WHERE NB = 1

Answer 2

You may want try WITH clause. Roughly...

WITH IPWITHMAXVLANID(IP, MAXVLAN) AS (
  SELECT i.IP, max(v.VLAN_ID)
   FROM IPS i 
   LEFT JOIN VLAN_IP_REL v_i ON i.ID_IP=v_i.ID_IP
   LEFT JOIN VLANS v ON v_i.ID_VLAN=v.ID_INSTANCIA
   GROUP BY i.IP
)
SELECT i.ID_IP, i.IP, i.FQDN, iml.MAXVLAN
FROM IPS i
INNER JOIN IPWITHMAXVLANID iml on i.IP = imp.IP

Hope this helps.