Simplify query by using COALESCE

postgres/queries/cumultative-balances.sql

@@ -4,5 +4,5 @@ SELECT COALESCE(ass.date, tra.date), COALESCE(ass.category_id, tra.category_id),
        COALESCE(tra.amount, 0)::decimal(12,2) as transactions
 FROM assignments_by_month as ass
 FULL OUTER JOIN transactions_by_month as tra ON ass.date = tra.date AND ass.category_id = tra.category_id
-WHERE (ass.budget_id IS NULL OR ass.budget_id = @budget_id) AND (tra.budget_id IS NULL OR tra.budget_id = @budget_id)
+WHERE COALESCE(ass.budget_id, tra.budget_id) = @budget_id
 ORDER BY COALESCE(ass.date, tra.date), COALESCE(ass.amount, tra.amount);