产品信息表product_information(product_id,product_name,product_description,category_id,list_price,min_price),查询list_price为空的产品的数量,下列sql语句正确的是:
A.
select count(list_price) from product_information where list_price is null;
B.
select count(list_price) from product_information where list_price = null;
C.
select count(nvl(list_price,0)) from product_information where list_price is null;
D.
select count(distinct list_price) from product_information where list_price is null;