Select zero when no results in date range - mysql

Question

I have a query that I am using to pull back the total costs per months for the previous 6 months of data. The issue I need to solve is when there is no records for a specific month, nothing is returned and only 5 months are shown.
I need to modify this query to always show the 6 months, even when there is no data for a specific month but I am unsure how to accomplish this.
select sum(cost),
CASE
WHEN MONTH(collection_date) = 1 THEN 'January'
WHEN MONTH(collection_date) = 2 THEN 'February'
WHEN MONTH(collection_date) = 3 THEN 'March'
WHEN MONTH(collection_date) = 4 THEN 'April'
WHEN MONTH(collection_date) = 5 THEN 'May'
WHEN MONTH(collection_date) = 6 THEN 'June'
WHEN MONTH(collection_date) = 7 THEN 'July'
WHEN MONTH(collection_date) = 8 THEN 'August'
WHEN MONTH(collection_date) = 9 THEN 'September'
WHEN MONTH(collection_date) = 10 THEN 'October'
WHEN MONTH(collection_date) = 11 THEN 'November'
WHEN MONTH(collection_date) = 12 THEN 'December'
ELSE 'NULL'
END AS datemodified
from invoices
WHERE collection_date >= DATE_SUB(now(), INTERVAL 5 MONTH)
GROUP BY MONTH(collection_date)
ORDER BY collection_date asc;
Sample of the results with an empty month
COST Datemodified
300 September
200 November
200 December
Desired output
COST Datemodified
0 August
300 September
0 October
200 November
200 December

Answer 1

You can create fake month data and join your invoices table to it. Try this:
SELECT SUM(cost), months.name AS datemodified
FROM (SELECT 1 AS num, 'January' AS name
UNION SELECT 2, 'February'
UNION SELECT 3, 'March'
UNION SELECT 4, 'April'
UNION SELECT 5, 'May'
UNION SELECT 6, 'June'
UNION SELECT 7, 'July'
UNION SELECT 8, 'August'
UNION SELECT 9, 'September'
UNION SELECT 10, 'October'
UNION SELECT 11, 'November'
UNION SELECT 12, 'December') months
LEFT JOIN invoices.collection_date = months.num
WHERE collection_date >= DATE_SUB(NOW(), INTERVAL 5 MONTH)
GROUP BY MONTH(collection_date)
ORDER BY collection_date ASC;
However, that gives you all the 12 months. To get only the 6 last months, you need to dynamically generate your fake month data:
SELECT SUM(cost),
CASE num WHEN 1 THEN 'January'
WHEN 2 THEN 'February'
WHEN 3 THEN 'March'
WHEN 4 THEN 'April'
WHEN 5 THEN 'May'
WHEN 6 THEN 'June'
WHEN 7 THEN 'July'
WHEN 8 THEN 'August'
WHEN 9 THEN 'September'
WHEN 10 THEN 'October'
WHEN 11 THEN 'November'
WHEN 12 THEN 'December'
END AS datemodified
FROM (SELECT MONTH(NOW()) AS num
UNION SELECT MONTH(DATE_SUB(NOW(), INTERVAL 1 MONTH)) AS num
UNION SELECT MONTH(DATE_SUB(NOW(), INTERVAL 2 MONTH)) AS num
UNION SELECT MONTH(DATE_SUB(NOW(), INTERVAL 3 MONTH)) AS num
UNION SELECT MONTH(DATE_SUB(NOW(), INTERVAL 4 MONTH)) AS num
UNION SELECT MONTH(DATE_SUB(NOW(), INTERVAL 5 MONTH)) AS num) months
LEFT JOIN invoices.collection_date = months.num
WHERE collection_date >= DATE_SUB(NOW(), INTERVAL 5 MONTH)
GROUP BY MONTH(collection_date)
ORDER BY collection_date ASC;