SELECT exp.Month , exp.ExpenseCount , coll.CollectionCount
FROM
(SELECT
    TO_CHAR(ExpenseDate, 'YYYY-MM') as Month,
    COUNT(1) as ExpenseCount
    FROM Expenses
    GROUP BY TO_CHAR(ExpenseDate, 'YYYY-MM')
) exp ,
(SELECT
    TO_CHAR(CollectionDate, 'YYYY-MM') as Month,
    COUNT(1) as  CollectionCount
    FROM Collection
    GROUP BY TO_CHAR(CollectionDate, 'YYYY-MM')
) coll
WHERE exp.Month = coll.Month ;

MONTH     | EXPENSECOUNT | COLLECTIONCOUNT
2017-11           1              1
2017-12           2              3