OCP-1Z0-051 补充题库 第30题 oracle中的NULL值

一、原题

View the Exhibit and examine the description for the CUSTOMERS table.



You want to update the CUST_CREDIT_LIMIT column to NULL for all the customers, where

CUST_INCOME_LEVEL has NULL in the CUSTOMERS table. Which SQL statement will

accomplish the task?

A.
UPDATE customers

   SET cust_credit_limit = NULL

 WHERE CUST_INCOME_LEVEL = NULL;
B.
UPDATE customers

   SET cust_credit_limit = NULL

 WHERE cust_income_level IS NULL;
C.
UPDATE customers

   SET cust_credit_limit = TO_NUMBER(NULL)

 WHERE cust_income_level = TO_NUMBER(NULL);
D.
UPDATE customers

   SET cust_credit_limit = TO_NUMBER(' ', 9999)

 WHERE cust_income_level IS NULL;

答案: B

二、题目翻译

下面是 CUSTOMERS表的结构.

CUSTOMERS表中，把CUST_INCOME_LEVEL列中有null值的用户的CUST_CREDIT_LIMIT列设置为null,下面哪个SQL能完成这个任务?

三、题目解析

在oracle中，找出null值，要使用 is null,而不能用=null,而设置值的时候，可以使用=null,所以B选项正确。