postgreSQL Query - Count column values matching on two columns

Question

I need help creating an SQL query to count rows broken out on two separate columns.

This is the DDL for my Tables:

CREATE TABLE Agency (
  id SERIAL not null,
  city VARCHAR(200) not null,
  PRIMARY KEY(id)
);
CREATE TABLE Customer (
  id SERIAL not null,
  fullname VARCHAR(200) not null,
  status VARCHAR(15) not null CHECK(status IN ('new','regular','gold')),
  agencyID INTEGER not null REFERENCES Agency(id),
  PRIMARY KEY(id)
);

Sample Data from the Tables

AGENCY
id|'city'
1 |'London'
2 |'Moscow'
3 |'Beijing'

CUSTOMER
id|'fullname'      |'status' |agencyid
1 |'Michael Smith' |'new'    |1
2 |'John Doe'      |'regular'|1
3 |'Vlad Atanasov' |'new'    |2
4 |'Vasili Karasev'|'regular'|2
5 |'Elena Miskova' |'gold'   |2
6 |'Kim Yin Lu'    |'new'    |3
7 |'Hu Jintao'     |'regular'|3
8 |'Wen Jiabao'    |'regular'|3

I want to count the new customers, regular customers and gold_customers by city.

I need to count separately for ('new','regular','gold'). Here is the output I want:

'city'   |new_customers|regular_customers|gold_customers
'Moscow' |1            |1                |1
'Beijing'|1            |2                |0
'London' |1            |1                |0

Answer 1

I was struggling with the same a couple of weeks ago.
This is what you need.

SELECT 
  Agency.city,
  count(case when Customer.status = 'new' then 1 else null end) as New_Customers,
  count(case when Customer.status = 'regular' then 1 else null end) as Regular_Customers,
  count(case when Customer.status = 'gold' then 1 else null end) as Gold_Customers 
FROM 
  Agency, Customer 
WHERE 
  Agency.id = Customer.agencyID 
GROUP BY
  Agency.city;

Answer 2

You could group on city, and then sum the number of statuses in each city:

select  city
,       sum(case when c.status = 'new' then 1 end) as New
,       sum(case when c.status = 'regular' then 1 end) as Regular
,       sum(case when c.status = 'gold' then 1 end) as Gold
from    customer c
join    agency a
on      c.agencyid = a.id
group by
        a.city