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    <p>Here are three different ways to do it. All depend on generating a time series then scanning it. One uses subqueries to aggregate data for each month. One joins the table twice against the series with different criteria. An alternate form does a single join on the time series, retaining rows that match <em>either</em> start or end date, then uses predicates in the counts to further filter the results.</p> <p><code>EXPLAIN ANALYZE</code> will help you pick which approach works best for your data.</p> <p><a href="http://sqlfiddle.com/#!12/99c2a/9" rel="nofollow">http://sqlfiddle.com/#!12/99c2a/9</a></p> <p>Test setup:</p> <pre><code>CREATE TABLE accounts ("id" int, "signup_date" timestamp, "cancel_date" timestamp); INSERT INTO accounts ("id", "signup_date", "cancel_date") VALUES (1, '2012-01-13 00:00:00', NULL), (2, '2012-01-15 00:00:00', '2012-02-05'), (3, '2012-03-01 00:00:00', '2012-03-20') ; </code></pre> <p>By single join and filter in count:</p> <pre><code>SELECT m, count(nullif(date_trunc('month',a.signup_date) = m,'f')), count(nullif(date_trunc('month',a.cancel_date) = m,'f')) FROM generate_series( (SELECT date_trunc('month',min(signup_date)) FROM accounts), (SELECT date_trunc('month',greatest(max(signup_date),max(cancel_date))) FROM accounts), INTERVAL '1' MONTH ) AS m INNER JOIN accounts a ON (date_trunc('month',a.signup_date) = m OR date_trunc('month',a.cancel_date) = m) GROUP BY m ORDER BY m; </code></pre> <p>By joining the <code>accounts</code> table twice:</p> <pre><code>SELECT m, count(s.signup_date) AS n_signups, count(c.cancel_date) AS n_cancels FROM generate_series( (SELECT date_trunc('month',min(signup_date)) FROM accounts), (SELECT date_trunc('month',greatest(max(signup_date),max(cancel_date))) FROM accounts), INTERVAL '1' MONTH ) AS m LEFT OUTER JOIN accounts s ON (date_trunc('month',s.signup_date) = m) LEFT OUTER JOIN accounts c ON (date_trunc('month',c.cancel_date) = m) GROUP BY m ORDER BY m; </code></pre> <p>Alternately, using subqueries:</p> <pre><code>SELECT m, ( SELECT count(signup_date) FROM accounts WHERE date_trunc('month',signup_date) = m ) AS n_signups, ( SELECT count(signup_date) FROM accounts WHERE date_trunc('month',cancel_date) = m )AS n_cancels FROM generate_series( (SELECT date_trunc('month',min(signup_date)) FROM accounts), (SELECT date_trunc('month',greatest(max(signup_date),max(cancel_date))) FROM accounts), INTERVAL '1' MONTH ) AS m GROUP BY m ORDER BY m; </code></pre>
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