Choose two.
Examine this SQL statement:
UPDATE orders o
SET customer_name =
(SELECT cust_last_name
FROM customers
WHERE customer_id=o.customer_id);
Which two are true?
A) The subquery is executed before the UPDATE statement is executed.
B) All existing rows in the ORDERS table are updated.
C) The subquery is executed for every updated row in the ORDERS table.
D) The UPDATE statement executes successfully even if the subquery selects multiple rows.
E) The subquery is not a correlated subquery.
Answer：BD
(解析：这道题考的就是关联子查询，类似题目以前有考过。实验证明 orders 的表所有行都会被更新到，但是不满足条件的是不会具体赋值，至少有更新的动作。
关联子查询：
1、 先执行主查询，对于主查询返回的每一行数据，都会造成子查询执行一次
2、 然后子查询返回的结果又传给主查询
3、 主查询根据返回的记录做出判断
)