merge into 相关操作

今天看ocp题，有一个新的发现：

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

当匹配上 的时候竟然可以执行两种操作；经过实验发现，这两种操作针对的是相同的行(两表匹配上的行)，其余行不考虑；