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...
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 :)
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'
