Choose the best answer
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:這道題的難點在題目的提問上,題目的意思是哪個查詢能夠檢索到 price 爲空的產品數量。由於 list_price 本身爲空,count 函數處理的方式是忽略,所以要用 nvl 函數來處理。)