How to set default value from mysql join interval yearmonth - mysql

Question

I have problem with my query. I have two tables and I want join them to get the results based on primary key on first table, but I missing 1 data from first table.
this my fiddle
as you can see, I missing "xx3" from month 1
I have tried to change left and right join but, the results stil same.
So as you can see I have to set coalesce(sum(b.sd_qty),0) as total, if no qty, set 0 as default.

Answer 1

You should cross join the table to the distinct dates also:
SELECT a.item_code,
COALESCE(SUM(b.sd_qty), 0) total,
DATE_FORMAT(d.sd_date, '%m-%Y') month_year
FROM item a
CROSS JOIN (
SELECT DISTINCT sd_date
FROM sales_details
WHERE sd_date >= '2020-04-01' - INTERVAL 3 MONTH AND sd_date < '2020-05-01'
) d
LEFT JOIN sales_details b
ON a.item_code = b.item_code AND b.sd_date = d.sd_date
GROUP BY month_year, a.item_code
ORDER BY month_year, a.item_code;
Or, for MySql 8.0+, with a recursive CTE that returns the starting dates of all the months that you want the results, which can be cross joined to the table:
WITH RECURSIVE dates AS (
SELECT '2020-04-01' - INTERVAL 3 MONTH AS sd_date
UNION ALL
SELECT sd_date + INTERVAL 1 MONTH
FROM dates
WHERE sd_date + INTERVAL 1 MONTH < '2020-05-01'
)
SELECT a.item_code,
COALESCE(SUM(b.sd_qty), 0) total,
DATE_FORMAT(d.sd_date, '%m-%Y') month_year
FROM item a CROSS JOIN dates d
LEFT JOIN sales_details b
ON a.item_code = b.item_code AND DATE_FORMAT(b.sd_date, '%m-%Y') = DATE_FORMAT(d.sd_date, '%m-%Y')
GROUP BY month_year, a.item_code
ORDER BY month_year, a.item_code;
See the demo.