PostgreSQL query including WITH subquery

Question

From the following table (named status), I need to extract town codes having a status of '01' by the end of year 2015. The column status_date stores the date at which a town changed its status.

gid  | town      | status  | status_date
-----+-----------+---------+-------------
1    | 86001     | 00      | 2000-01-01
2    | 86001     | 01      | 2016-03-01
3    | 86002     | 01      | 2000-01-01
4    | 86003     | 00      | 2000-01-01
5    | 86003     | 01      | 2015-03-01
6    | 86003     | 02      | 2015-09-01

I can achieve that with the following query which is a bit long:

WITH tab AS (SELECT town, MAX(status_date) FROM status GROUP BY town)

  SELECT 
    t.town

  FROM tab t
    LEFT JOIN status s ON t.town = s.town AND t.max = s.status_date 

  WHERE t.max < '2016-01-01' AND s.status = '01' ; 

The result is:

town   
-------
86002

Any idea on how to make this query simpler? Is the WITH essential?

---

To create the table for testing:

CREATE TABLE status (gid serial NOT NULL, town CHARACTER VARYING(5), status CHARACTER VARYING(2), status_date DATE) ;

INSERT INTO status (town, status, status_date) VALUES
  ('86001', '00', '2000-01-01'),
  ('86001', '01', '2016-03-01'),
  ('86002', '01', '2000-01-01'),
  ('86003', '00', '2000-01-01'),
  ('86003', '01', '2015-03-01'),
  ('86003', '02', '2015-09-01') ;

Answer 1

You can do this with distinct on:

select s.*
from (select distinct on (s.town) s.*
      from status s
      where s.status_date < '2016-01-01'
      order by s.town, s.status_date desc
     ) s
where status = '01';

This query will get the latest status for each town up to the end of 2015. The outer query then selects those that are 01.