How to subtract a field value total in one table from a field value in another table in Oracle

There are now two tables, a table field amount is the invoice amount, B table field Rev is the payment amount, the two tables are associated with the field ID, and you need to subtract the fields amount from table A to the field Rev of B, but the invoice represented by table A May correspond to the collection amount for multiple B tables. How to subtract the amount of table A from the number of Rev values of the corresponding B-table, and get the amount of the unpaid receivables

Select a.ID, (NVL (a.amount,0)-NVL (b.rev,0)) Result
From A,
(select B.id,sum (B.rev) REV
From b
Group BY b.ID) b
where a.id=b.id (+);

How to subtract a field value total in one table from a field value in another table in Oracle