Re: need nelp with aggregate functions

Howdy,

 

It is quite simple, the joins you used would output a long list of

pairs (client,computer), and, as many people has more than one computer, many clients will appear repeated, but the COUNT keyword will count them more than once though.

 

The DISTINCT keyword prevents one client from appearing more than once...

 

It's basically that...

 

Hope this helped

 

Best,

Oliveiros

 

----- Original Message -----

From: Another Trad

To: Oliveiros C,

Cc: pgsql-sql@postgresql.org

Sent: Wednesday, November 18, 2009 5:37 PM

Subject: Re: [SQL] need nelp with aggregate functions

ok, I did: SELECT count(DISTINCT c.cliente_id) as qtd_client,count(cm.cm_id) as qtd_computers
GREAT. It works.
Please, explain me why and how it works, I wanna learn and do by myself next time :)

2009/11/18 Oliveiros C, <oliveiros.cristina@marktest.pt>

Try substituting the SELECT count(c) as qtd_client,count(cm) as qtd_computers

 

by

 

SELECT count( DISTINCT c.cliente_id) as qtd_client,count(/* put here the primary key of the computer table */ ) as qtd_computers

 

Then tell me if it output what you want

 

Best,

Oliveiros

 

----- Original Message -----

From: Another Trad

To: pgsql-sql@postgresql.org

Sent: Wednesday, November 18, 2009 4:55 PM

Subject: [SQL] need nelp with aggregate functions

The DB structure is in attachment.
I with the number of clients and the number of computers that have processors with "manufacturer" = "INTEL" and "speed" = "2GB"
I am trying:

select count(c) as qtd_client, count(cm) as qtd_computers

  from cliente c
  inner JOIN computer cm on (c.cliente_id = cm.cliente_id)
  inner join processor p on (cm.processor_id = p.processor_id)
inner join speed s on (s.speed_id = p.speed_id)
INNER JOIN manufacturer m on (p.manufacturer_id=m.manufacturer_id)



where m.manufacturer = 'INTEL'
and s.speed = '2GB'

but is not working
anyone can help me?

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