Esto no es facil. Recibí ayuda de este hilo generate days from date range y lo combinó con su consulta.
Entonces, la idea era obtener la lista de fechas de los últimos 7 días y luego unir estas fechas con una cantidad estática de 0 a la consulta que tiene y finalmente sumarlas. Esto podría usarse para cualquier rango de fechas, solo necesita cambiarlos en ambas consultas
select
t1.purchase_date,
coalesce(SUM(t1.amount+t2.amount), 0) AS amount
from
(
select DATE_FORMAT(a.Date,'%Y-%m-%d') as purchase_date,
'0' as amount
from (
select curdate() - INTERVAL (a.a + (10 * b.a) + (100 * c.a)) DAY as Date
from (select 0 as a union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7 union all select 8 union all select 9) as a
cross join (select 0 as a union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7 union all select 8 union all select 9) as b
cross join (select 0 as a union all select 1 union all select 2 union all select 3 union all select 4 union all select 5 union all select 6 union all select 7 union all select 8 union all select 9) as c
) a
where a.Date BETWEEN NOW() - INTERVAL 7 DAY AND NOW()
)t1
left join
(
SELECT DATE_FORMAT(purchase_date, '%Y-%m-%d') as purchase_date,
coalesce(SUM(amount), 0) AS amount
FROM transactions
WHERE purchase_date BETWEEN NOW() - INTERVAL 7 DAY AND NOW()
AND vendor_id = 0
GROUP BY purchase_date
)t2
on t2.purchase_date = t1.purchase_date
group by t1.purchase_date
order by t1.purchase_date desc
