51、(12-10)choose the best answer:

Evaluate the following SQL statement:

SQL> SELECT cust_id, cust_last_name

FROM customers

WHERE cust_credit_limit IN

(select cust_credit_limit

FROM customers

WHERE cust_city ='Singapore');

Which statement is true regarding the above query if one of the values generated by the subquery is NULL?

A) It ignores the NULL value and generates output for the other values produced by the subquery.

B) It produces an error.

C) It generates output for NULL as well as the other values produced by the subquery.

D) It executes but returns no rows.

Answer：A

(解析，子查询中如果有出现某行值为 null，则忽略该行，其它的正常处理，原来 051 的考题。

类似的语句：

select ename from emp where empno in (select mgr from emp);

ENAME

FORD

BLAKE

KING

JONES

CLARK