Subquery using WITH

query.sql

WITH cohort as (
  select author_id from articles where datetime >= '2015-01-01' and datetime <= '2015-01-31'
), return_rate as (
  select date_trunc('month', datetime) AS month, count(*) FROM articles WHERE author_id IN (SELECT author_id FROM cohort) AND datetime >= '2015-02-01' GROUP BY month ORDER BY count ASC
)
SELECT to_char(month, 'Mon YYYY'), count from return_rate;

schema.sql

CREATE TABLE articles (
    id          integer CONSTRAINT firstkey PRIMARY KEY,
    author_id   integer NOT NULL,
    datetime    timestamp without time zone
);

INSERT INTO articles VALUES (1, 1, '2015-01-01');
INSERT INTO articles VALUES (2, 1, '2015-02-01');
INSERT INTO articles VALUES (3, 1, '2015-03-01');