Examine the description of the PRODUCT_INFORMATION table:
Name NULL? Type
PROD_ID NOT NULL NUMBER(2)
PROD_NANE VARCRAR2 (10)
LIST_PRICE NUMBER(6,2)
Which query retrieves the number of products with a null list price?
A) SELECT (COUNT(list_price) FERM Product_intormation WHERE list_price=NULL;
B) SELECT Count(nvl( list_price,0) )FROM pruduct_information WHERE list_price is null;
C) SELECT COUNT(DISTINCT list_price) FROM produet_infomation WHERE list_price is nulll.
D) SELECT COUNT(list_price) from proceduct_information where list_price is NULL;
Answer：B
(解析：这道题的难点在题目的提问上，题目的意思是哪个查询能够检索到 price 为空的产品数量。由于 list_price 本身为空，count 函数处理的方式是忽略，所以要用 nvl 函数来处理。)
原文：https://blog.51cto.com/13854012/2453928