Merge table in Oracle with delete condition refering to source table - sql

Question

For the following question it is said that the answer should be C. But I think the correct answer is Answer D as NOT MATCHED block inserts all unmatching records to target table. Can anybody explain this?
Thank you.
Q)View the Exhibit and examine the data in ORDERS_MASTER and MONTHLY_ORDERS tables.
Evaluate the following MERGE statement:
MERGE INTO orders_master o
USING monthly_orders m
ON (o.order_id = m.order_id)
WHEN MATCHED THEN
UPDATE SET o.order_total = m.order_total
DELETE WHERE (m.order_total IS NULL)
WHEN NOT MATCHED THEN
INSERT VALUES (m.order_id, m.order_total);
What would be the outcome of the above statement?
A. The ORDERS_MASTER table would contain the ORDER_IDs 1 and 2.
B. The ORDERS_MASTER table would contain the ORDER_IDs 1,2 and 3.
C. The ORDERS_MASTER table would contain the ORDER_IDs 1,2 and 4.
D. The ORDERS_MASTER table would contain the ORDER IDs 1,2,3 and 4.
Answer: C

Answer 1

The correct answer is indeed C, this is because the source of the merge operation is the monthly_orders table, which only contains two records with order_id 2 and 3 respectively.
Think about what will happen for each of these records:
For order_id = 2, because this id exists in the order_master table, we'll execute the MATCHED part of the merge statement, updating the order_total to 2500. Since the quantity for this record is not NULL, the DELETE won't do anything.
For order_id = 3, again, the id exists in the order_master table, so we execute the MATCHED part of the merge statement, updating the order_total to NULL and then issuing a DELETE on order_master for the row we just updated because the quantity on monthly_order is NULL.
This leaves us with order_id 1, 2 and 4, which matches answer C.
Code
CREATE TABLE orders_master (
order_id NUMBER(1) NOT NULL
,order_total NUMBER(10) NULL
)
/
CREATE TABLE monthly_orders (
order_id NUMBER(1) NOT NULL
,order_total NUMBER(10) NULL
)
/
INSERT INTO orders_master (order_id, order_total) VALUES (1, 1000)
/
INSERT INTO orders_master (order_id, order_total) VALUES (2, 2000)
/
INSERT INTO orders_master (order_id, order_total) VALUES (3, 3000)
/
INSERT INTO orders_master (order_id, order_total) VALUES (4, NULL)
/
INSERT INTO monthly_orders (order_id, order_total) VALUES (2, 2500)
/
INSERT INTO monthly_orders (order_id, order_total) VALUES (3, NULL)
/
MERGE INTO orders_master o
USING monthly_orders m
ON (o.order_id = m.order_id)
WHEN MATCHED THEN
UPDATE SET o.order_total = m.order_total
DELETE WHERE m.order_total IS NULL
WHEN NOT MATCHED THEN
INSERT VALUES (m.order_id, m.order_total)
/
COMMIT
/
SQL> select * from orders_master
2 /
ORDER_ID ORDER_TOTAL
---------- -----------
1 1000
2 2500
4