Question

I am wondering why the below query is not retrieving the difference between two aggregate functions(sum) properly ?

SELECT 
    epa.status as Status,
    sum(equip_port_usage.total_nbr_ports) as Total,
    sum(equip_port_usage.NBR_PORTS_ASSIGNED) as Used,
    sum(equip_port_usage.NBR_PORTS_ASSIGNED)
      - sum(equip_port_usage.NBR_PORTS_ASSIGNED) as Difference
  FROM site_inst
  INNER JOIN site_attr_settings
     ON site_attr_settings.site_inst_id = site_inst.site_inst_id
  INNER JOIN epa on epa.site_inst_id=site_inst.site_inst_id
  INNER JOIN equip_inst ON equip_inst.site_inst_id=site_inst.site_inst_id
  INNER JOIN equip_port_usage
     ON equip_port_usage.equip_inst_id=equip_inst.equip_inst_id
WHERE site_inst.SITE_HUM_ID = 'CLEUS'
GROUP BY epa.status;

The output is like below

Ok        303876  10276     0
Faulty     19044    644     0
Reserved   19872    672     0

I am expecting it to be

Ok        303876  10276  293600
Faulty     19044    644   18400
Reserved   19872    672   19200

Answer 1

It’s a copy and paste failure in your code.
You’re calculating

sum(equip_port_usage.NBR_PORTS_ASSIGNED)
 - sum(equip_port_usage.NBR_PORTS_ASSIGNED)

which is always 0.

Try:

SELECT 
    epa.status as Status,
    sum(equip_port_usage.total_nbr_ports) as Total,
    sum(equip_port_usage.NBR_PORTS_ASSIGNED) as Used,
    sum(equip_port_usage.total_nbr_ports)
     - sum(equip_port_usage.NBR_PORTS_ASSIGNED) as Difference
  FROM site_inst
  INNER JOIN site_attr_settings
     ON site_attr_settings.site_inst_id = site_inst.site_inst_id
  INNER JOIN epa ON epa.site_inst_id=site_inst.site_inst_id
  INNER JOIN equip_inst ON equip_inst.site_inst_id=site_inst.site_inst_id
  INNER JOIN equip_port_usage
     ON equip_port_usage.equip_inst_id=equip_inst.equip_inst_id
 WHERE site_inst.SITE_HUM_ID = 'CLEUS'
 GROUP BY epa.status