Fill missing data points in `GROUP_CONCAT`

I need to generate data to build some charts. The current results have missing data points and I'd like them to fill them with 0's. Data is stored in MySQL 8. Simplified data sample & query fiddle here . The query I currently have is

SELECT
	currency,
	GROUP_CONCAT(volume) AS volume
FROM (
	SELECT
		DATE(t.created_at) AS created_at,
		t.currency AS currency,
		SUM(t.amount) AS volume
	FROM
		transactions AS t
	WHERE (t.created_at BETWEEN @start AND @end)
	GROUP BY
		created_at,
		currency
	ORDER BY
		created_at,
		currency) r
GROUP BY
	currency

which creates this result set: | currency | volume | |----------|---------------------------------------------| | AUD | 27553.52,13395.20,18349.51,3773.29,... | | BRL | 272.45,... | | CAD | 14738.08,7372.58,5926.08,7877.14,... | | CHF | 320.00,27.00,47.00,27.00,... | | EUR | 888.62,2806.27,4445.30,805.93,... | | GBP | 48588.64,37266.79,27275.01,13981.08,... | | MXN | 10.00,16298.00,1900.00,... | | SEK | 497.00,497.00,1491.00,... | | USD | 374660.85,347793.84,523608.81,839710.22,... | Where I need help: - How can I fill the mising data points with 0? - Let's assume the worst and at some point there are no transactions for any of the currencies for a day (or multiple days). How can I fill those missing data points? I've read quite a few posts about WITH RECURSIVE and calendar tables but I can't wrap my head around it. I'd appreciate any help/pointers. Thank you! ### Update 1 @Akina's answer basically does what I asked for (thank you!) **but**: the query takes *ages* to complete. The transactions table currently holds ~4m rows of data. A monthly resultset averages to roughly ~270k rows. Amogst others, there are indexes on currency, created_at and a compound index for created_at,currency. ### Update 2 Something is off with my indexes. If I LEFT JOIN transactions AS t FORCE INDEX(created_at) ... then the query completes in ~15s, regardless if I set the date range to a month or 6 months.