Can't find out expected result from Left Join Where & group by - mysql

Question

I have two table receivable & receive.
table: receivable
tranId roll month amount
1 1111 October-2019 10
2 1112 October-2019 10
3 1113 October-2019 10
4 1114 October-2019 10
5 1115 October-2019 10
Table: receive
tranId roll month amount
1 1111 October-2019 10
2 1111 September-2019 10
3 1113 October-2019 10
4 1114 October-2019 10
5 1115 October-2019 10
6 1116 October-2019 10
7 1117 October-2019 10
8 1118 October-2019 10
9 1119 October-2019 10
10 1120 October-2019 10
In both table month columns are string & table receive roll+month column is unique. i want to create a receivable & receive statement report of a student (Where roll=1111) from those table like.
Expected result:
month rcvamount rcvvmonth rcvvamount
---------------------------------------------------------
October-2019 10 October-2019 10
---------------------------------------------------------
September-2019 10 - -
my query is:
SELECT receive.month, sum(receive.amount) AS rcvamount, receivable.month AS rcvvmonth,
receivable.amount AS rcvvamount
FROM receive
LEFT JOIN receivable ON receive.month = receivable.month
WHERE receive.roll = 1111
GROUP BY receive.month
and resutl is:
month rcvamount rcvvmonth rcvvamount
----------------|-------|----------------------------------
October-2019 | 50 | October-2019 10
----------------|-------|----------------------------------
September-2019 10 - -
revamount in October-2019 should be 10 from receive table.

Answer 1

You can aggregate before joining. Or use union all and aggregate:
SELECT month, SUM(ramount) AS rcvamount,
SUM(raamount) AS rcvvamount
FROM ((SELECT month, amount as ramount, null as raamount
FROM receive
WHERE roll = 1111
) UNION ALL
(SELECT month, NULL as ramount, amount as raamount
FROM receiveable
WHERE roll = 1111
)
) r
GROUP BY month;
One nice feature of UNION ALL is that it handles missing data in either table.