sql - In PostgreSQL, get a minimum value if there is none -

sorry if title did not match actual question. i'm not sure how phrase this.

in postgres, have users can @ station. stations numbered 1 5. have table users , has column station. if user @ station, station value number of station, if not, null. here's how find number of users @ each station:

select station, count(users) users     users     station not null     group station order station; 

this yields:

 station | users  ---------+---------        1 |     4        2 |     1        3 |     1 

but that's not really true. there 0 users on station 4 , 5. how can address that? use query view calculate other things, need stations there. don't need table stations, far know: i'm not going use them anywhere else.

so how account unused stations? need make table them , reference stations table users table or there simpler way?

assuming have table stations, can handle left join:

select s.station, count(u.station) users stations s left join      users u      on s.station = u.station group s.station order s.station; 

without such table, can use generate_series():

select s.station, count(u.station) users generate_series(1, 5) s(station) left join      users u      on s.station = u.station group s.station order s.station;