Insert zero values for unexisting groups in Redshift - sql

Question

I'm writing a simple query on Amazon Redshift as follows:
SELECT EXTRACT(year FROM created_at) AS year,
EXTRACT(month FROM created_at) AS month,
member_id,
COUNT(*) as pageviews
FROM TABLE
GROUP BY year,
month,
member_id
ORDER BY year,
month,
member_id
This gives me the following result as an example:
year month member_id pageviews
2015 1 100 29
2015 2 100 22
2015 3 100 178
2015 4 100 34
2015 1 200 56
2015 3 200 16
Here's the result I would like to have:
year month member_id pageviews
2015 1 100 29
2015 2 100 22
2015 3 100 178
2015 4 100 34
2015 1 200 56
2015 2 200 0
2015 3 200 16
2015 4 200 0
In the result above, notice the additional rows with zero pageviews.
How do I get this result? Any help would be much appreciated.

Answer 1

Use a cross join to generate the rows and then a left join to bring in the data:
SELECT EXTRACT(year FROM created_at) AS year,
EXTRACT(month FROM created_at) AS month,
m.member_id,
COUNT(t.member_id) as pageviews
FROM (SELECT DISTINCT EXTRACT(year FROM created_at) AS year, EXTRACT(month FROM created_at) AS month FROM TABLE) ym CROSS JOIN
(SELECT DISTINCT member_id FROM TABLE) m LEFT JOIN
TABLE t
ON EXTRACT(year FROM created_at) AS month = ym.year AND
EXTRACT(month FROM created_at) AS month = ym.month AND
t.member_id = m.member_id
GROUP BY ym.year, ym.month, m.member_id
ORDER BY ym.year, ym.month, m.member_id;
This assumes that all year/month combinations are included in the table.
If you have other tables that are better sources for members and the dates, try them -- that may be faster than SELECT DISTINCT.