Avere una tabella di tutti i mesi e poi left join alla tua tavola:
SELECT MONTHNAME(m.month) AS mName,
MONTH(m.month) AS mOrder,
ifnull(sum(amount),0) AS total_num
from months m
left join income i
on m.month = i.date
GROUP BY mOrder
ORDER BY mOrder DESC
Se non vuoi creare un months tabella allora puoi:
(select STR_TO_DATE('01/01/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/02/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/03/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/04/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/05/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/06/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/07/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/08/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/09/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/10/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/11/2016', '%d/%m/%Y') as month union
select STR_TO_DATE('01/12/2016', '%d/%m/%Y') as month)
