Select count of users registered in each month

Having a very simple table:

CREATE TABLE users (
  name VARCHAR(50) NOT NULL,
  registered TIMESTAMP NOT NULL,
  CONSTRAINT users_pk PRIMARY KEY (name)
);

How to select the number of user registrations in each month e.g.

Jan 2010 - 19,
Feb 2010 - 0,
Mar 2010 - 7

Answer

To get those zero entries you need to join across generate_series:

select 
  to_char(gen_month, 'Mon YYYY'),
  count(name)
FROM generate_series(DATE '2010-01-01', DATE '2010-04-01', INTERVAL '1' MONTH) m(gen_month)
LEFT OUTER JOIN users 
ON (registered BETWEEN gen_month AND gen_month + INTERVAL '1' MONTH - INTERVAL '1' DAY)
GROUP BY gen_month;

You can make it a bit prettier by using date_trunc, but then you can’t use a regular b-tree index on registered:

select 
  to_char(gen_month, 'Mon YYYY'),
  count(name)
FROM generate_series(DATE '2010-01-01', DATE '2010-04-01', INTERVAL '1' MONTH) m(gen_month)
LEFT OUTER JOIN users 
ON ( date_trunc('month', registered) = date_trunc('month', gen_month) )
GROUP BY gen_month;

If you want to pretty-print your output exactly as you wrote it, you could replace the SELECT clause with:

SELECT format('%s - %s', to_char(gen_month, 'Mon YYYY'), count(name))