day2作业

1
select product name, regist_date
from product
where regist_date > "2009-4-28";
2
选出product表中，purchase_price为空的所有行

选出product表中，purchase_price不等于空的所有行

选出product表中，purchase_price不等于空的所有行
3
select product_name, sale_price, purchase_price
from product
where sale_price - pruchase_price > 50